#3 Misfire and Variable Cylinder Compression

Grant Lasson

Grant Lasson

TJ Enthusiast
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Aug 15, 2019
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Sandy, Utah
A check engine light showed up just as I needed to register this year. The code showed #3 misfire. I pulled the spark plugs. #3 had some carbon but nothing dramatic. If anything, it showed less wear than the other five. I was shocked at the wear on my Autolite XP985 Iridium plugs in less than 50K miles. I don't really have a closeup camera but if I can figure something out I'll post pictures. I pulled the fuel pump relay and the injector fuses and did a compression test.
Here's what I got in two tries:

#1: 95, 107; gap 0.052"
#2: 150, 150; 0.052"
#3: 97, 108; 0.040"
#4: 142, 134; 0.066"
#5: 150, 155; 0.061"
#6: 72, 80; 0.059"

The plugs started at 0.035" gap.

I tried to do a leak down compression test but the crank would move off TDC with 100 PSI inlet. (Should I just replace all but one spark plug?)

I've got new spark plugs and the injector insulator is arriving today for the "infamous" #3 cylinder misfire though I don't really see heat soaking as the issue right now. We're still in winter.

Any advice on what I'm looking at? or what I should look at next? Odds that this engine is toast? Only has about 150K miles on it.
 
Leak down will let you test the valve seal too. Regardless of piston location.i would check that before condemning the motor. A valve job isn't a hard thing to do
 
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JKP said:
Put the trans in 4th, parking brake on. Engine won't turn.

It's possible you have carbon buildup on the valves. Do you ever run it above 3500 RPM?
Click to expand...

I run between 3 and 3.5K quite a bit. I'm sure I run it over 3.5K but not on a sustained basis.
 
I managed to get the leak down test done today after work. Cylinders 2,4, &5 are all pretty good at 6%, 6%, and 0% loss. #1, #3, and #6 are a different story at 53%, 53%, and 70% respectively. Those leaks seem to be exhaust leaks for the most part. It does seem like the #2 intake might be leaking a bit. I didn't really pickup anything coming through the PCV/oil intake.

I did put a scope in each cylinder. They seemed pretty clean relative to my direct injection Mazda.

It's tempting now to get the valve train rebuilt.

The more cautious option would be to do a wet compression test to further isolate cylinder/ring/piston issues from valve train issues. I'd also like to try the technical bulletin process. I don't have their valve spring compression tool though and it seems pretty different from most similar tools available. I suppose if I drop a valve into the cylinder I could pull the head.
 
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You don't have to use their specific valve spring tool, although it might be slightly more of a pain to do it with a regular one.

If you follow the procedure, the valve can't completely fall into the cyl as the piston is at TDC:
Bring number one (#1) cylinder piston to top dead center using the mark on the
crankshaft front dampener/pulley. This step is important to prevent the possibility
of the exhaust valve from falling completely into the cylinder.

You can also used compressed air to hold the valves in place.

Personally, I'd get a can of something like Techron complete fuel system cleaner and take it out and drive it like I stole it, and then recheck the numbers.

You can also steam clean the carbon by slowly trickling water down the throttle body at idle.
 
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Update: I drove it pretty hard (holding 4500 RPMs for up to 30 seconds 5-6 times) and then I ran Berryman Intake Valve and Combustion Chamber Cleaner through it and let it heat soak for two hours. Then I repeated the hard driving cycle. It seemed to help. Here are the results.

Compression
#1: 95, 107 went to 150, 150
#3: 97, 108 went to 92, 93
#4: 142, 134 went to 135, 140
#6 72, 80 went to 150, 151

Leak down:
#1: 81/38 for a 53% loss went to 80/79 for a 1% loss
#3: 80/38 for a 53% loss went to 80/26 for a 67% loss
#4: 80/75 for a 6% loss went to 80/66 for a 18% loss
#6 80/24 for a 70% loss went to 80/79 for a 1% loss

Overall, I'm pretty happy with that progress. #1 and #6 have returned to full function. #3 and #4 seem to have their compression losses through the exhaust. I replaced all the spark plugs (Autolite XP985) and added the fuel injector insulation to #3.

I cleared the codes and now need to drive it at least 100 miles to see if the misfire code recurs. If it doesn't, I'm good to get it registered. If it does then I've got to make some bigger choices. I think that would start with a new Clearwater top end. Right now they're only $650 with free shipping. It's hard to imagine getting it rebuilt locally for less.
 
If you seats and valves aren't bad but just dirty you could pull the head and do lapping compound?

If i pulled the head i would want it to have a quick pass on a machine to make sure its flat though.they could hot tank it for you too
 
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Grant Lasson said:
Update: I drove it pretty hard (holding 4500 RPMs for up to 30 seconds 5-6 times) and then I ran Berryman Intake Valve and Combustion Chamber Cleaner through it and let it heat soak for two hours. Then I repeated the hard driving cycle. It seemed to help. Here are the results.

Compression
#1: 95, 107 went to 150, 150
#3: 97, 108 went to 92, 93
#4: 142, 134 went to 135, 140
#6 72, 80 went to 150, 151

Leak down:
#1: 81/38 for a 53% loss went to 80/79 for a 1% loss
#3: 80/38 for a 53% loss went to 80/26 for a 67% loss
#4: 80/75 for a 6% loss went to 80/66 for a 18% loss
#6 80/24 for a 70% loss went to 80/79 for a 1% loss

Overall, I'm pretty happy with that progress. #1 and #6 have returned to full function. #3 and #4 seem to have their compression losses through the exhaust. I replaced all the spark plugs (Autolite XP985) and added the fuel injector insulation to #3.

I cleared the codes and now need to drive it at least 100 miles to see if the misfire code recurs. If it doesn't, I'm good to get it registered. If it does then I've got to make some bigger choices. I think that would start with a new Clearwater top end. Right now they're only $650 with free shipping. It's hard to imagine getting it rebuilt locally for less.
Click to expand...

I had the same thing happen(po301) to my 2002,did the ac delco top engine cleaner through the brake vacuum hose,let sit overnight .then did the 4599 for 15 seconds,run really good now.
 
Success! I drove over 100 miles and the check engine light did not come on. I was able to get it registered yesterday.

I'm happy to have that done but I'm still going to try and rotate at least valves 3 and 4 and do the process again. I'm still getting quite a bit of exhaust popping when I decelerate. I'm guessing that it's valves 3 and 4 not sealing completely.

Thanks for your help. I'll post an update if I make further progress.
 
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Grant Lasson said:
Success! I drove over 100 miles and the check engine light did not come on. I was able to get it registered yesterday.

I'm happy to have that done but I'm still going to try and rotate at least valves 3 and 4 and do the process again. I'm still getting quite a bit of exhaust popping when I decelerate. I'm guessing that it's valves 3 and 4 not sealing completely.

Thanks for your help. I'll post an update if I make further progress.
Click to expand...

Do one more treatment when it is hot,let it soak a few hours.then do the 4500 and I also bought a case of this supposed to be good stuff I run a can every tank,there is a video on utube of a guy cleaning his pistons.

IMG_4204.png
 
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