To avoid taking over Kerry's AFM Problem topic, I thought I would start a fresh one for my misfire (?) issue.
At the West coast champs, I had problem with my engine hesitating (even from idle in the paddock) but worse than that after 10mins of racing it would lose power and become very flat under load.
Below is a video of the engine sound when the power loss occurred.
As you can hear it sounds like a misfire. We did a quick compression check at the track and all cylinders were right around 155 PSI (is this in the expected range for a '88 engine?). We also pulled the plugs and found the #3 plug was very black, whilst the rest seemed normal. This lead us to believe it could be the problem cylinder. We did a quick spark check of all plugs and it looked like all have a strong spark.
At the track we changed out the DME, rotor, cap, spark leads, spark plugs, as well as fixed the connector on the #3 injector (which lost the retaining clip).
Obviously we never resolved the issue. However when I was taking the car of the trailer at home I heard a loud ticking noise, which we could not hear whilst in the busy paddock.
Here is a recording of the ticking I can here. It is much louder just as the engine starts and gets quieter within a 10 seconds. Could it be a bad lifter, which is compressing and gets quieter as oil pressure builds and pumps it up? My thinking is this is something that could get worse again when the engine heats up and the oil gets thinner?
Other suggestions at the track were a broken valve spring, vacuum leak and broken AFM.
My next steps are to do vacuum pressure testing, AFM voltage test, injector ohm check (noid light too, if I can get hold of one), leak down test.
If all these check out, I hope my plan to pull the head and have it and the injectors rebuilt.
The ticking sound is most likely a lazy lifter filling back up. It will work fine once it's filled up with oil.
The misfire sounds like one cylinder? If it occurred at 10 minutes in, it's a temperature caused problem or something shook loose? Check that injector connector. The spark plug should be very different looking from the others if the injector isn't shooting fuel.
It probably wouldn't be a valve spring unless it happens at higher rpm. There are 2 valve springs per, so 1 would be enough to close the valve at low rpm.
Not a vacuum leak as this would be a prob all the time.
Cheap Noid is a GM part. Writing on mine says, "OTC 7602 GM PFI."
Not likely the AFM as this would be a prob upon start up.
Please say hi to Sir Richard and your mum.
Tim Comeau
SoCal 944 Spec #22 since Feb 2003.
Let's keep building it!
I would guess a dirty injector not closing all the way and leaking fuel. I would pull the rail and turn the engine over to see the pattern on the injectors. It would be pretty obvious if it is as bad as it sounds.
Also a soft lifter on an intake valve can cause low lift on that valve which would show up as a rich plug since there is not enough air getting past the valve compared to the fuel injected.
If the lifter is getting lazy, it is best to just replace it right?
I have ordered a noid light and will pull the injectors out this weekend. can anyone recommend a good injector cleaning service. I have seen the following:
The misfire sounds like one cylinder? If it occurred at 10 minutes in, it's a temperature caused problem or something shook loose? Check that injector connector. The spark plug should be very different looking from the others if the injector isn't shooting fuel.
One of the plugs we remove from the track was very black (#3 cylinder), so this would add up to a rich mixture, either not always getting spark or not getting the expected air.
tcomeau wrote:
It probably wouldn't be a valve spring unless it happens at higher rpm. There are 2 valve springs per, so 1 would be enough to close the valve at low rpm.
Not a vacuum leak as this would be a prob all the time.
Not likely the AFM as this would be a prob upon start up.
There is definitely something wrong at idle too, if you remember we never got it to sound smooth. It always had a hiccup (gurgling exhaust) and hesitation.
It is probably more than one thing. So I am just going to eliminate them one at a time.
tcomeau wrote:
Please say hi to Sir Richard and your mum.
Will do, thanks for all your help and guidance at the track, it was great meeting you.
In my tool box I carry a spare injector plug/harness which I can plug into a suspected injector. I pull the rail and jumper the fuel pump to supply pressure. First off there should be no fuel spraying or dripping from any injector at this point. Then I plug in my testing plug to each injector and supply +12V and Ground to the test plug to observe the spray pattern from each injector. It is not a pulse but a constant spray. I then pull the power to see how well it closes. Testing each one will give you an idea how well they are all working and if there are any spraying different. Takes all of 5 mins to do this.
I would suggest picking up a set of injectors, have them cleaned and tested and have them in your spares box.
