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L98 Misfire problem


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FelixP

Addict

Wed 27 Sep 17 16:46

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


--------------------
1989 Corvette C4 L98 6 speed ZF with Z51 & FX3

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stevel98

Club Member

Wed 27 Sep 17 17:03

Re: L98 Misfire problem


Hi..

Is the poundage of the new injectors same as the stock ones? If not, that could be a problem for the ECU.

I would have changed to coil too, could be that yours is breaking down as it heats up.

Scratchchin


--------------------
RIP Gunny.. Hoo Ra, Semper Fi, Carry on..

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chevrolet

Club Member

Wed 27 Sep 17 18:58

Re: L98 Misfire problem


Well, I am definitely no L98/TPI expert. Presumably, the theory is that putting the car on the dyno may have caused/raised the problem? e.g running the engine at/or near max revs for an extended period?

Presuming you have looked at the Haynes manual? In mine for an 84-87, it lists 10 possible causes why "Engine misses throughout driving speed range"


--------------------
AKA "Silver C6" on Pistonheads

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peterguest

Club Member

Mon 02 Oct 17 12:57

Re: L98 Misfire problem


Could be the knock sensor, mine packed up out of the blue and caused a misfire across a fair part of the rev range.

You can check it as it does throw up a code, which I can't remember, but I'm sure some knowledgeable person here will know what it is.

straight forward to replace but a bit of a pain to get at, on my 91 had to remove a heat shield as its just under the exhaust manifold on the passenger side of the block as I recall.

rgds

pete

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FelixP

Addict

Tue 03 Oct 17 12:20

Re: L98 Misfire problem


Thanks guys

I don't have a Haynes. Chevrolet - Yes that is my theory. I think I'm in the midst of a lesson learned here!

Steve - Injectors are factory spec and caused no issues in the last 12 months since fitting

Peter - I don't have any codes so don't think it's the knock sensor?

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 sound it makes that you can hear in the end of the video.


--------------------
1989 Corvette C4 L98 6 speed ZF with Z51 & FX3

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roppa440

Club Member

Tue 03 Oct 17 14:36

Re: L98 Misfire problem


Could a vacuum hose have come off or split?

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chevrolet

Club Member

Tue 03 Oct 17 14:47

Re: L98 Misfire problem


Must admit, don't totally understand the sounds in the video compared to my "stock" '85 . Presume yours will sound different anyway due to the headers (?) Are the background "banging" noises the misfire? Haynes manual (not expensive):

https://haynes.com/en-gb/chevrolet-corvette-84-96-haynes-rep...

Don't know if you can see this attachment (extract from Haynes manual)?



Last edited by chevrolet on Tue 03 Oct 17 15:12; edited 2 times in total. [13 %]
--------------------
AKA "Silver C6" on Pistonheads

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FelixP

Addict

Thu 05 Oct 17 22:59

Re: L98 Misfire problem


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.



Last edited by FelixP on Thu 05 Oct 17 22:59; edited 1 time in total. [4 %]
--------------------
1989 Corvette C4 L98 6 speed ZF with Z51 & FX3

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stevel98

Club Member

Fri 06 Oct 17 12:53

Re: L98 Misfire problem


What kind of leads are you using?

I had problems with another car using silicon/carbon leads from Halfords (orange and yellow) which kept failing and eventually the replacements failed too. I ended up visiting a lawnmower shop and bought a (cheap) roll of black/copper HT lead and caps which solved all the misfiring.

These days, I'm using Taylor products specifically their copper leads and distributor cap with brass fittings.

https://www.summitracing.com/int/parts/tay-76240/overview/ye...

If I'm doing ignition these days, I prefer to source all the components from the same manufacturer if I can.


--------------------
RIP Gunny.. Hoo Ra, Semper Fi, Carry on..

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FelixP

Addict

Fri 06 Oct 17 13:31

Re: L98 Misfire problem


All of my parts are genuine AC Delco


--------------------
1989 Corvette C4 L98 6 speed ZF with Z51 & FX3

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