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Bosch Spark Plug War

New 7/5/05

1) I have an intermittent low coolant light, comes on after getting on it occasionally. What is causing it? Sensor?

2) Ive replaced pretty much every coil except one coil in the front bank, and it has new bosch +4 plugs but the car still has a miss. A pretty good miss actually when cruising at 60mph and the converter locked up. Seems to be more right after my IMRC went and wired it full open, but there was still a miss before hand.

3) Im running pretty rich, according to my friends that are following me, and if you get on the car say from a dead stop up to 60 or so, you can smell it inside the car.

4) What is the part number for the IMRC?, currently im wired full open, but would like to fix that.

If anyone has any thoughts or advice on this I would appreciate it much.

Jon Martinucci 97 ES 98 SF

Unless you are running NOS or a blower I would recommend stock plugs.


If you can smell it (exhaust) when you get on the car it is likely the hoses to the DPFE on the back of the surge tank. An EGR code is in your future. but if you look back there carefully with a light from the passenger side I bet you can see that at least one of the hoses isn't connected between the exhaust lines and the DPFE, the exhaust is actually being sucked through your cabin air filter.

The low coolant light is quit possibly the sensor going to heaven (or the magnet on the float in the tank rather). Good luck on the miss. I'd tell you to rip those +4's out but others I am sure would disagree. I've never personally met a Ford that likes Bosch plugs, which suits me just fine.


I have been smelling the richness for a long time now. Still no code. My coolant sensor is fried, too, methinks. It comes on after I get on it for a bit.


Chuck the +4s and get Motorcraft plugs. This may very well fix the miss AND the richness (likely just unburnt fuel passing through due to the POS bosches not igniting the mixture.)

Replace the coolant tank & low coolant sensor.

The IMRC can be had dirt cheap from junkyards. It's fully interchangeable with the Duratec SLO unit. Just use longer bolts to mount it to the valve cover.


Or you COULD just unplug the coolant sensor, that works too.....

Adam Phillips

Id rather purchase a new IMRC, I have a friend at the parts department at my local ford dealer.

Part number anyone?



(to Dan)

Interesting statement and I am not saying I understand the TOO completely but: Plugs not firing causing the to rich mixture. I have a set of mixture gauges on each bank and when my #7 coil was going south I was getting both P-0171 and P-0175 codes along with what I considered erratic but fairly normal rapid flipping between rich & lean cycling at about 1 second intervals. When the P-0307 finally showed up the gauge on the front bank went crazy flipping so fast I couldn't hope to keep count of the cycling. In any case after replacing the #7 coil I stopped getting the P-0701 and now only get the P-0705. It always made sense to me that the unburnt fuel would cause an excess of oxygen in the exhaust resulting in the P-0171/P-0175 indications but I pretty much have confirmed it on my car. Interesting it took over a year and 10,000 miles for the P-0171 to develop into a P-0307 though. Of course I still have to prove this by either trial and error on the rear bank or just replacing all 4 (I have two of them from Murray's but not so sure I spring for the other 2 before the convention) since it probably won't honor me with a code in the near future.

On a separate issue, after test driving Chuck's car I was baffled by the very noticeably better power delivery then mine when it was NA. Chuck confirmed that it had a AWL2 which oddly enough was the same code on the one I drove to Tulsa that also had similar power. Is it just a fluke or are all the AWL2's doing something different to make more free-reving type power?

Carter Fuji

Bahhh--Been running them for quite some time, no better, no worse than any other plug. Pulled them after the 1st remake STB and they were clean as a whistle. Still doing the happy dance in #2 rebuild.

Larry E

(to Carter)

Don't know about that Bud - original SW9 was AWL2 and was a slug. Slowest GIII in the world, well next to Ricks and Scotts pre rowboat days. :}

Larry E

You can believe whatever you want. The fact is, you are LUCKY if the Bosch plugs work for you. There is documented PROOF that for Taurus owners (SLOs included), there is a statistical probability of greater than 30% that Bosch plugs will FAIL within 20K miles, and a large portion of that 30% will fail almost immediately.


Carter - FWIW IMO there are 3 distinct flavors or V8 SHO performance in the many SHO's I've driven. All 3 years seem to have an equal chance of being either Slow, Medium, or Fast. I used to think the 96's favored fast a little bit more frequently, but lately I've driven a number of SLOWWWW 96's. IMO, again, it's in the Engine, not the CPU and it seems to be a "You git whatca' git" kind of thing. I DID change a couple of specs in SW9, however, but Don't take My advice, I use Bosch +4 plugs all the time. Don't believe the SW9 timeslips either.

Next subject - IT'S NOT THE PLUGS! ! ! ! ! ! I keep watching this Monkey See, Monkey Do show. Would you folks get over it? No, you all probably WON'T. (To Be Great is to be Misunderstood - Emerson) It's a Coil, as usual, and Jon is smelling unburnt fuel. It's the COILS - ALWAYS THE COILS.

Scott's DPFE concept is a possibility, but I think I would have noticed that right off the bat when I put in Jon's replacement wiring harness last winter. And Scott - while I agree with your statement that the Bosch's didn't like my FORD engine (302) mainly due to plug location, They LOVE my YAMAHA Engines, Like they love MOST Import Engines.

Coolant sensor - unplug it and be done with the damn thing. They Suck.

IMRC # - F6DZ - 9L492 - AA , I believe.

Flame suit Deployed....


I'm going to TRY to draw a picture here. I'm going to try to use small words too.

THIS IS NOT A FORD HEAD. The plug Does NOT come in Sideways into the combustion chamber like a FORD Head. This is a Center Mount Plug.

Pretend you are on the piston looking up at the sparkplug end in the combustion chamber.

In a traditional Ford head the sparkplug end is sticking in SIDEWAYS, and a Bosch +4 type plug will fire a vertical shrouded spark about 1/2 the time, from your point of view. hence, the intermittent Miss on Ford Engines. A standard plug will fire from one side of the head towards the other - this sideways spark is more desireable because it exposes the most spark to the fuel charge coming up from the piston face.

On V8 SHO Heads, (Yamaha - NOT Ford) the plug is in the center, pointing straight down at the piston. A traditional plug shrouds the spark from the fuel charge in this case, you cannot SEE the spark from straight down on the piston face. A Bosch +4 plug ALWAYS has a sideways spark kernel in this situation, offering a more desireable SIDEWAYS Spark that is exposed to more fuel than a vertical spark will be.

As for outnumbered - meh - From an earlier post today - "To Be Great is to be Misunderstood" - Emerson. We eventually got over the Whole "Earth is Flat" thing too, but more than one right minded fellow was flamed in the process. SOMEBODY Has to continue the education or we will cease to improve.

Flame away....

Eric Lehmann

Well that was a bit more concise than my answer. Dan, I'm curious, on what data are you basing your stats on? More curious than anything else. IF what you say is true, and as everyone knows, if something bad is going to happen to an engine (motor) it will happen to mine, why is SW9 such a happy camper? Just wondering.


So the 3.0L Duratec head is not a Ford head because the plugs are centered and mounted vertically?



AT LEAST a 36.25% failure rate in our sample size of 80 vehicles

17.5% failed immediately

Another 18.75% that didn't fail immediately failed within 20K miles (premature for a platinum plug)

There are still another 31.25% that are working so far, but haven't accumulated 20K miles yet. So they are "up for grabs" statistically, an could inflate that 36.25% failure rate even higher.

Still want to gamble?


I just don't see where the Bosch +4 is an improvement in any way. A typcial plug will shroud the spark from the fuel for appox 20% of the circle. A +4 will, by design, shroud about 80+% of the spark from the circle. I just don't get the advantage. I think they say it has more platinum, but only the center electrode has platinum, and THERE is the problem with the +4. Motorcraft plugs (at least originally) had both the center and ground electrode in Platinum to let the live the recommended 100,000 miles. So I think that only a true dual platinum plug will be able to last the distance. I am pretty sure Larry changes engines a lot sooner than that, so he may not know how long a Bosch +4 will last period (sorry Lar, low blow, but I am feeling frisky today!). :)

Bosch also claims a "the longest spark". Excuse my non-electrical engineer brain, but I am pretty sure the ignition system controls h ow LONG the spark lasts, and that no matter how many ground electrodes you have, you can't make it last longer than there is juice? NO?

They say that the four electrodes provide optimum access to the air/fuel mixture. I still say it just shrouds more of the spark from the mixture. The spark will only go to one electrode, so there almost no better chance that the spark will actually find the mixture than with a regular plug. where the spark is visible around more than 300 degrees of the plug.

Finally, I know of nobody that can prove any performance gain with these plugs, so if all else is equal, I will go with what seems to work the best. Since we have few complaints on the stock plugs, and lots of complaints on Bosch.

Just my thoughts.


Your theory doesn't hold water.

The Motorcraft plugs don't cause problems, even on a "Ford" engine with "Ford" heads, even when indexed 180* out of the proper position, so the ground electrode shields the spark EVERY SINGLE TIME.

Dan to Eric

Do all four tangs draw fire or only one? Does having four tangs quadruple the chances of a fouled plug? IIRC, only one tang draws a spark under running conditions and if any of these are fouled the plug is history. On two stroke mx bikes, some are notorious for eating plugs, the better plugs have finer electrodes and a some what different shape to the tang, less surface area, which seems to offer a little more resistance to fouling, but at $2 vs $8, not worth the extra money in my opinion. I know this is no two stroke, but just considering the possibilities. The fuel charge will be in motion throughout the combustion area, squish area and all the factors it brings into play, flame front propagation, I don't know just how much of a difference a plug with an open shot to the electrode will have vs a standard plug. There may be a difference, maybe some motors more than others, but I would guess the majority of the time there would be little to no difference. No fire suit needed. No slam intended. I know this list can go ape over the littlest thing, but how about some interesting debate, maybe we all can learn something.

k mier

Which is why, on any OHV engine (Chevy, Ford, whatever), the best way to go is to index the plugs, so the electrode is to the short side, and the "open" part of the plug faces the bulk of the CC.

Now, on a Bosch +2 or +4, you have either 2 or 4 shrouded areas. In a central-plug OHC engine, you won't have the issues of an OHV wedge engine, but the electrodes still shroud the spark a bit.

IMHO, the +2 and +4 plugs are primarily a marketing ploy. The only benefit that I can see is that, for a 100K plug change, at least ONE of the electrodes can maintain a tighter gap, and get better performance if you wait the full 100K.

For a performance engine that is regularly maintained, I truly can't see how a +2 or +4 plug is of any benefit whatsoever.

The stock plugs (1) work well, and (2) have come down in price over the years. Since there have been "issues" with non-stock plugs (admittedly, mostly on the V6 SHOs), I can't see any reason for risking an issue in using gimmicky plugs.

If you have not experienced problems, that's great. I don't see where they will gain any performance over the stock plugs, and since that is the case, I would never use them.

Ron Porter

Only one will fire. Electricity follows the path of least resistance. Once the gap that was the tightest wears down, it will find the next tightest gap.

Ron Porter to K Mier

I agree that they have no benefit.

They do not, however, shield the spark. I just replaced the plugs on my wife's M Roadster and it has factory installed Bosch 4 electrode copper plugs. I was rather interested in seeing that the center electrode was even with all 4 side electrodes. I also changed the plugs on my 328is and it has Bosch with 2 electrodes laid out the same way.

If you look at http://www.boschusa.com/AutoParts/SparkPlugs/PlatinumPlus4/# and click on the tip, you will see what I mean. The ground electrodes don't got past the end of the center.

Paul L Fisher

That's correct, Ron. The easiest path is the one that fires. If that path grows from wear or is fouled, then the NEXT path takes over. This is a main reason why I prefer these. You can foul these plugs three times before that Hole goes dead - it is not about the first 10k, its about the LAST 10k. Most SHO Owners are VERY reluctant to pull their surge tank to perform "Regular" Maintenance. These plugs, IMHO, will get those owners a little bit farther for the slightly higher expense. I've pulled a set of +4's with about 60k on them and it is clear visually from the electrode that ALL 4 tangs had been receiving fire on all 8 plugs. I'm trying to attach a photo close-up but my camera isn't the best at point blank images. Notice the 4 tangs are all similarly heated and the 4 spark kernel marks on the center electrode.

Eric Lehmann

Old Bosch Plug2.jpg (11971 bytes)

(RE TCCA link)

That's weighted with Duratecs. I was wondering about specific numbers for 3.4's

Larry E

Well Dan - I Don't Know the 3.0 V6. And I wasn't talking about it when I said TRADITIONAL Ford Head. Are you familiar with the Windsor series of Engines (~1962 to 2001)? THAT is what I was referencing - just like the kid with the 94 Mustang has in the TCCA thread. I also went to that thread and noticed only ONE V8 SHO Owner, Some guy from Philadelphia....... huh.

MY Cross section of 80 cars begs to differ with the Drink Box and Wet Nap crowd over on TCCA (Sorry Bob, It's true) I've had ALOT of problems with Idling and Jon's car. Lets start with the Mouse Eaten wiring loom hanging in My Garage. That car is STILL a work in progress and NOTHING that car does suprises me much anymore. Recently it has a CEL and a "Magic" code which doesn't like to appear on regular code readers for some reason. Do I think that car is De-bugged completely? Not on a Dare. Do I think Changing the plugs will help - Not on your life or mine.

 I stay in frequent contact with most of My camweld customers and have ZERO failures related to these sparkplugs, but dozens of Intermittent Coil problems which 96 and 97 CPU's just CAN'T seem to throw codes on a coil problem.

 Speaking of Codes - The P030X codes that always get thrown are NOT referencing a sparkplug or coil. How many Fuel Injectors have you changed because of that code? It is ALWAYS the COILS. P035X series is an example of a COIL Code - How often do you see those? Almost never. I put the +4's in based on my belief that they stand a chance of lasting longer and tolerating fouling because of their multiple spark path possibilities, not whether they perform better on a dyno. This is my "Gamble" - that my customer may get one more season out of his/her plugs than if we just went stock. Also, There is NO SUCH THING as even CLOSE to a 100,000 mile plug - Talk about Myths that should be put to bed.

60k is a stretch for ANY plug, IMNSHO.

Never said the Ford plugs were bad, just think the +4's are slightly better and I don't charge extra for them. Slightly better is fine with me to justify the little extra.

Still Gambling with a clear conscience

Eric Lehmann

That is kind of what I thought regarding power, but the only thing that has remained curious to me is that the only part that has stayed constant over the life of my car was the AWL1 that I had in the firewall. The Original engine was a '97 that was swapped to a crate "96 then when the piston burned I switched it to a "99 bottom end that Kirk gave me. All three configurations ran about the same but nowhere close to what James Aldrich's car ran like or like Chuck's new one which are both AWL2's. Just thought I was onto something.

By the way, I never had problems with the Bosch Platinum I ran in my SHO though of course they haven't been in there since adding the Vortech. And as for the coils, well I think they are really a PIA. Though when I finally did have one fail completely and I replaced it I could certainly tell a difference from even before it finally threw out the code.

Carter Fuji

That's why I feel that these plugs are for those who are lazy on maintenance.

For proper performance, change the regular plugs at no later than 50K.

Same deal as more frequent oil/tranny fluid/intake cleaning/brake fluid/ps fluid changes....you do these for peak performance, not the lowest common denominator as specified from the factory.

Ron Porter

Duratecs are still central-plug DOHC engines.

Interesting tat there would be failures, as any central-plug OHC engine should not be as sparkplug-sensitive as an inefficient OHV wedge-head.

Ron Porter

And therein is what I believe to be E1's point.  - Larry

I agree. The engineers have alot of years testing what is right and what is wrong. If you are going to have a car and it is basically stock, stick with factory stuff. If you are going to heavily modify an engine then you will have to go to a different route. Personally I can do a complete plug change in about 3 hours or a little less depending on how hard I work at it. Then again I do have more resources than most in my garage to make my work easier. On a personal note I don't like Plat. plugs, every since my T-bird was eating them about every 20K or so. I like the adjustability that I can do with a copper plug, but then yet I have no life and change when needed.


Indeed this is the entire concept of the multi ground electrode plug. Anything else is hocus pocus. The plugs should not be in the car longer than 50k anyway.

I can't see any reason not to use the Motorcraft plug unless it is an emergency situation and you had to get another plug immediately and then I would go Autolite. The Motorcraft plugs were designed for the engine and are not expensive or hard to get.

Paul (to Ron)

I subscribe to the Ron Porter theory of how the engine was broke in is how it will run.



I had looked at that page before replying. Looking at it, there is no other conclusion than the +4 shields the spark from the air/fuel mixture, at least from the sides, nearly 100% of the time. There is ground electrode almost ALL the way around the plug. Where on a one ground plug, the spark is clearly exposed almost 300 degrees or more around the center electrode. I just don't see any possible performance advantage, and it IS just a marketing ploy like 6 blades on a razor or colored stripes in a tire.

Plus they keep implying that they have FOUR platinum electrodes, when only the center one is such, and the ground electrodes are all mild steel.

The number of people that have without prompting reported problems with these fancy plugs are enough to make me recommend only the Motorcraft plugs.

Don Mallinson

I from the Bosch USA page.
Features Benefits
Revolutionary firing technology featuring multiple spark paths Delivers the longest, most powerful spark for ultimate engine performance
More pure platinum vs. Bosch Platinum For smoother acceleration, peak fuel efficiency and maximum engine power
Four ground electrodes Provides optimum access to the air/fuel mixture for the most efficient combustionůplus a longer performance life
Electrode gap is factory set Provides ease of installation - never requires adjustment


1. Multiple spark paths, as one wears down the other which have maintained their preset gap will take over but still only one electrode at a time.
2. More platinum that the standard Bosch Pt.  No reference to any competitors.  Maybe the standard Bosch plug is
3. Four ground electrodes provide optimum access????  Pure marketing hype.  Refer to argument 1.
4. Never requires adjustment,  Thank goodness as it is almost impossible to do at home or in a shop.  What if you drop one?

Which is why, on any OHV engine (Chevy, Ford, whatever), the best way to go is to index the plugs, so the electrode is to the short side, and the "open" part of the plug faces the bulk of the CC.

Now, on a Bosch +2 or +4, you have either 2 or 4 shrouded areas. In a central-plug OHC engine, you won't have the issues of an OHV wedge engine, but the electrodes still shroud the spark a bit.

IMHO, the +2 and +4 plugs are primarily a marketing ploy. The only benefit that I can see is that, for a 100K plug change, at least ONE of the electrodes can maintain a tighter gap, and get better performance if you wait the full 100K.

For a performance engine that is regularly maintained, I truly can't see how a +2 or +4 plug is of any benefit whatsoever.

The stock plugs (1) work well, and (2) have come down in price over the years. Since there have been "issues" with non-stock plugs (admittedly, mostly on the V6 SHOs), I can't see any reason for risking an issue in using gimmicky plugs.

If you have not experienced problems, that's great. I don't see where they will gain any performance over the stock plugs, and since that is the case, I would never use them.

Ron Porter

I also have changed my plugs to Bosch Platinum +4 last week. First 2 days after that, car ran great. The next day, idling was surging and i felt a soft misfire. 2 days after that, seems all cylinders are misfiring. I can smell unburnt fuel after that. Reading this topic, I decided to check the plugs and sure enough, it was black with soot. When car is cold, it rans normal but with a slight idle surge. After 15 to 20 minutes driving it is back, the car would hesitate and when stopped, it feels like its being strangled.

Valentin N. Salgado 

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