Mike J

Ok, the owner who now owns my old Targa has an issue with the car, and I thought i would ask around for opinions.

• The car exhibits a miss on WOT that you can feel from around 4000rpm up - the miss is not enough to cause CEL's, but it definitely slowing the car down
• The miss is erratic, it does clear up and then reoccur, its more happening than not at the moment
• I changed the plugs/rotors/caps, spark plugs were perfect, light grey, no carbon buildup evident
• Engine rebuild was 18,000 miles ago, new plug wires as well as total rebuild, very low oil consumption
• Car run strong and no-misses until 100 miles ago
• Cannot feel any misses unless WOT
• Computer scan shows Cylinder #3 misfire(s) - Weissach did this test
• Quick compression test on #2,#3, #5, #6 engine hot was well within 10% of each other and normal ranges, leak down test on those pistons was very close to 5% across the board
• The owner took the car to Weissach - they ran a number of tests (not exaclty sure what), and concluded that Piston #3 has heavy carbon on the piston crown - they visually checked this with a boroscope. However, compression numbers are even across the 4 cylinders we checked, including #3

I am not sure if Weissach checked the ignition system, I will check. However, their claim is interesting, what conditions would cause heavy carbonization ? The car is driven fairly hard, certainly not lugged.

Weissach also claims the MAF is showing more air flow than should be happening by a large margin - and they swapped out the MAF and then the DME with no change in reading. The implies some sort of air leak to me. I inspected the intake manifold and plastic air intakes, visually look OK but its hard to check them properly.

Suggestions on how to go forward? I have some ideas, but there are certainly more knowledgeable people on this list than moi.

Cheers,

Mike

ble2011

I'm no expert, but maybe a bad fuel injector in cylinder #3?

techman1

Mike,
Not sure if the twin plug could cover up 1 plug not firing, but maybe one of the new wires for #3 is bad?

Do you have a spare spark plug wire long enough to reach? My first check would be unhook, not remove, # 3 wire, and plug your spare in, routing it as best as possible to replace the one you unplugged. Then see if the miss is still there. If so, do the other distributor, same scenario.

Comparing lifter noise from adjacent cylinder, nothing strange at idle?

Is anything unique to cylinder #3 when you owned it?

Hmm. Carbonization of #3... Brawny? (Sorry Travis!)

Quadcammer

what brand of alt/fan belts are on the car?

distributor belt ok?

Mike J

Fan belts are Porsche - it has an RS pulley - and it was just replaced with no change
The ECU was swapped - no change
The MAF was swapped - no change
All the plugs are very clean and have the proper color - I pulled the old set out and replaced them, along with the caps/rotors - the old plugs, original when I rebuilt the car, were of perfect color
Distributor belt is good
Car sounds great, no unusual noises, no lifter noises
Nothing unique on Cylinder #3.
Spark plug wires replaced when I rebuilt the engine - so they have 18,000 miles.

As far as the fuel injectors go, I might swap #3 with another to see what happens.

If #3 plug wires were bad, why would the plugs have the proper color - usually misfiring plugs change their color and texture.

I think a hint may be that the MAF is showing way too much air volume - perhaps a leak somewhere. I have not checked the vacuum levels, I was thinking of a split intake runner.

The chase is on - he only brings in the car when we are doing the next tests, so this may take a while.

Cheers,

Mike

vincer77

Just a thought - how about running a bottle of Techron. Usually clears up the acceleration in my car.

As far as the MAF reading high, I can't see it being due to a leak. The MAF is upstream of the throttle body, therefore the pressure in the intake between the MAF and engine is negative. That could cause a leak in which case the MAF should read low.

geolab

Hi Mike, if you swap, this will totally mislead you.
Suppose the injector is the culprit, the miss will stay the same, and
you have to wait for the other piston top to build carbon so that
you deduce it is the injector.

Heavy black deposits on the piston dome are unburned carbon accumulated because of too rich a mixture, amongst other causes.
Too rich a mixture in one cylinder only, is logically, a defiance in the electric of
this same cylinder.
Moreover, an un burnt mixture is going to give a rich signal to the ECU
which in turn thinks it needs more air, thus more air through the MAF.
A trick in the 993 engine mngmt is that a defective or clogged injector will make the system run richer on
all other banks as well, and same MAF overflow data, happened to me.

But our cylinder #3 has two distinct plug cables and plugs
What controls both plugs at the same time ?
I would not omit the ignition module under the driver seat, from my list to check.
regards

Triple G

I think by swapping injectors to a different cylinder you can see if the misfires move from cylinder #3 to the cylinder you swap it with.

If there was a vacuum/air leak wouldn't the MAF read less air passing through it?

geolab: With a rich condition why would it cause a lean signal? It would be a rich signal. I agree that this would make the ECU think it needs more air causing the high MAF readings.

Is it possible to read the injector duty cycle to see if the 5 other injectors are working at a different rate than the cylinder #3 injector?

geolab

Stupid iPad , it thinks thus it changes words as it suits
Corrected initial post, thanks.
With the tester, Mike could check the misfire bank when swapping injectors,
but I responding in the thought that Mike does not have the tester.
Through the tester you could activate the injector on/off only, with
the engine off only.

Quadcammer

vacuum leak after the maf would cause a CEL for hitting a rich correction limit.

Stealth 993

Mike I don't think a vacuum leak would cause high MAF readings. If anything, it would show a lower MAF reading, as the engine would suck in air form the leak & not through the MAF.

I say flow test the injectors, might just be a bad/dirty/sticky injector. Also it would be nice to do some loging of the O2 at WOT. Also try to get the temps of the cylinders, & see if #3 is different.

Mike J

Hi George,

i have a PST-2 if that is what you mean by "tester". Not sure what you mean by "activate the injector on/off only, with the engine off only"?

Ok, so lets go with the premise to focus on the injector. Remember that Weissach was reading the codes for the Cylinder #3 misfire, I have not had a chance to hook up the PST-2 and check it out directly (yet). I also do not have a boroscope to check out the other cylinders.

Good ideas from everyone! I need to check with the owner to see when he can bring it over AND I have time to look at it - in the midst of a very large reno project right now...

Cheers,

Mike

geolab

Mike, with the pst2, things are easier now. I know you are going to
Swap injector #3, and initiate miss. Retest with pst2.

What I meant in the above sentence is that " your injector if faulty, is only
so Above 4000 rpm and wot" so you have to initiate the miss to pinpoint the injector.
And in answering the question of "Triple G": with the Pst2, you cannot read the duty cycle of the injector.
The only thing the PST2 can do with the injector itself, is turn it on or off, and the car engine has to be shut off.
ie : in the Pst2 menu, go to ECU - real values - injectors , and choose injector #3 with the F12 key, the press F8 to see that the injector opens, and again F8 to close it.
This will only work with engine shut.
But as I understood, the miss is happening above 4000rpm.
So testing the injector is going to give no fault probably under the above procedure.
Swapping injectors and triggering the miss and looking for misfire code in pst2 is the only way me thinks, to pinpoint injector, with a PST2, as all the guys said in this thread before...
Please note all errors when you read them, O2 sensors and others if any, as Stealth 993 said
But temp is only available for one cylinder in the Pst2 tester, number 3 BTW
Let's try to find culprit with minimal costs, as usual

JasonAndreas

The O2 sensor is ignored at WOT and even at some high load levels.

Stealth 993

The ECU might ignore the reading, but it's still getting data.