L98 Misfire problem
 

Hi all looking for some advice.

My 89 6 speed has 104k and I took it on the dyno since it was running sweet with the new headers. Now it has a misfire at all RPM. Please see attached video.

https://sendvid.com/tbdya808

I have recently changed:
Coolant temp sensor & connector
Plugs
O2
Leads
Bosch III injectors

I have put on new:
Cap
Rotor arm
Ignition control module

I have checked:
Plug wire routing
TPS voltage
MAF unplugged/plugged in - No difference
Injector resistance & fuses

Did your situation happen during the Dyno or after the Dyno run? If after how long after and anything happen that would make you think it contributed?

What was the max RPMS that you hit?

Revved to 52-5300 on 3 runs. Sounds like it was bogging down on the last run. Driving home I stopped for coffee and it was ok for 2 minutes but now it's missing regardless of temp. I'll check for a big vac leak tomorrow but when the injectors were done, upper plenum gaskets were too.

Hi

I would check the ignition coil and injector connectors, the coil output may be low. Another thing is check the injector harness connectors, there is just a thin metal strip from the wire in the injector harness connection.

Be it from previous owners that just tug the injector harness off the injectors, i had a misfire but the engine did shake on acceleration.
Took some time to find as when i flipped the injector harness over to test the connector 12 volt pulses all was fine, however was still missing. Replaced injectors , ignition leads , spark plugs, distributor cap, coil and distributor module ... no codes but ran like crap.

One night i had an idea, started the car from cold ran for 20 seconds and turned her off. Checked each header pipe and found one cool,
checked spark - ok. swapped injector harness from the one nearby (they batch fire so you can do that) turned out the fault went to the other cylinder. Replaced the harness injector connector and problem fixed, a crack in the small metal strip was enough to show 12 volt spikes during cranking on multimeter. Would not allow enough current to drive the injector open.

Its usually a simple problem, don't forget the knock sensor if it senses knocking it will retard the ignition a lot. the ecm will compensate but can get itself in knots, sometimes just disconnecting the battery or ECM connector near battery and resetting the ecm codes may help fix some rough running issues.

Good luck :cheers:

Put a piece of vacuum hose between the cap & ign wire. Start up engine, touch the hose with a grounded test light, this will kill that cylinder. Find the one that doesn't effect the rough idle.

Thanks for the suggestions chaps

I've tried swapping injector harnesses around, no change. Likewise tried pulling leads off the dizzy cap, seems to have less of an impact on 7 & 8.

I took the valve covers off and the rocker arms seemed consistent, I turned it over a few times and watched them. The springs feel tight too.

I have Hedman headers so I can't take the plugs out without taking them off which is a pain. Would I be jumping the gun to just drain the oil and see what's in the pan/filter? I really don't like the banging sound it makes that you can hear in the end of the video.

I just went through something similar in my race car. Turns out from sitting, the points on the wheel that signal the magnetic pickup were corroded and the mag pickup had trouble at higher RPM. Datalogging showed my timing was all over the place and I couldn't rev past 4K RPM without pinging and misfire. Dropped in a new distributor and all was good. I probably could have just rebuilt/cleaned the old one but time was not a luxury I had. I will eventually rebuild the old and keep it as a spare.

Got myself a Haynes manual.

Offset spanner did the trick on the plugs, cylinder 7 was black. Did a compression test and all cylinders were 175-180psi. Put it back together, ran ok for a few minutes then started missing again. With an IR temp gun I found the exhaust on cylinders 8&6 half that of the rest. Using a stethoscope I could hear all injectors pulsing. Happily no more banging sounds.

I'll order and ignition coil now and see how I get on.

Yeah i have to use a offset ring spanner to get my spark plugs out with the headman headers, you did fit new plugs didn't you? they are cheap or at least swap them from good running cylinders.

well now you know what cylinders have the misfire, start the engine with 8 and 6 spark plug leads disconnected run for a few seconds and check the spark plugs are wet from fuel. That way you know your injectors are working properly.

The misfire is between 7&8. Since it seems like a popular place for the headgasket to go, i ordered a block test but this came back ok. I had the car up to temp and it didn't change colour as coolant entered the expansion tank. I'll try and look into the injectors. I have spark on all the plugs and can hear the injectors pulse but perhaps they've blocked up?

7 and 8 are on opposite sides of the engine.

Oops, I mean 8&6

Headers=more noise

check your knock sensor

Id pull it and wrap Teflon tape on threads and loosely re install, or put a 90 degree fitting on it moving it different angle from headers

I finally got to the bottom of this and it was a cheap and easy fix!

When I changed the leads a few had got a little too hot near the headers despite having those socks on the boots. When I put new leads on, I didn't take the boots off to slide on the heatshields. Anyway, I started it up and now we have darker evenings, I saw one of the leads shorting against the header!

I'll have to run them very neatly. I was using Summit racing leads but had put new AC Delco ones on, I guess the insulation isn't as good, as they were further away from everything previous but still the spark jumped across. Thank you daylight savings! And a good lesson in checking & double checking the simple things :silly:

Just for info, I did that test and it did not show blown head gasket at number 7, but it did turn out to be bad gasket Don’t trust that test entirely. Mine is 89 also

Put in a new coil. Are 'leads' your term for spark plug wires? If they havent been changed I'd replace them as well.