c36 missfires at 4000rpm???????
 

Hey, I have a 1996 c36 with 140k miles. I changed the crankshaft pulley to a lightweight one. Now my mechanic said the car will misfire then it will go away due to the lightweight pulley, which it did. I put the stock crankshaft pulley and still no luck, so now it has the lightweight pulley back on it. Then I accidentally took out the ECU without disconnecting the battery. Then the misfires started to happen. I have checked the timing over and over, I changed the spark plugs and the ignition coils no luck. Any ideas from what may cause the misfire.

You talking in N or P ??????????????????????

while i drive

Any codes?????????

I would start with checking all of your main harness grounds. Isuzu's are horrible for this coming loose. The only thing I can think of is that your evap sensor might have something to do with it. The idling variance might be at fault tho.

<The only thing I can think of is that your evap sensor might have something to do with it. The idling variance might be at fault tho.
>


What?????

my check engine light was on and when I had the checked it said it was the egr valve. I changed that still missfire. I took it to another mechanic and he says the vacuum line the goes to the egr valve to the engine could be clauged up, he apparently has been working on C36 when they first came out and they have a problem like this.

As for codes, if i recall it said p0400, if that makes sense, give or take a zero at the end.

Sorry for my lack of knowledge but this car is driving me nuts already.

I have rebuilt the engine from ground up, and all these problems started to occur when I disconnected the ecu with out first disconnecting the battery.

So did you check the vacuum line to the EGR to see if it is clogged up?

BTW, I don't think unplugging the ECU with the battery connected would cause the problem, so you can stop beating yourself up over that IMHO.

You never got a misfire code?

All vacuum lines have been changed about 5-6k miles ago. I haven't yet cleaned the vacuum line, because I am trying to find the right tool so I won't cause anymore harm, like puncturing the line in a place I can't reach. I have had codes that say "random misfire" or "misfire on cylinder x"

I have changed Ignition coils, plugs, wire, cap, rotor, egr, and double checking timing.

What amazes me is that it happens at a certain rpm??????

Thanks for your input so far much appreciated.

Thank You

<<What amazes me is that it happens at a certain rpm??????>>

The car has a rev limiter @ 4K for torque converter protection.
Try 4K in N or P ... you will notice the car will misfire at the 4 K....if that is the same as you complaint, then take the car for a ride and try different gear selsctions and see if that clears it up at the same rpms...if yes, you may have a shorted NSS .
With foot on brake, see if starter will crank when car is in D.

Okay I tried revving it in neutral and park, the rev limiter stops it at 4k rpm and no misfire, in 2nd gear it misfires along with 3rd.

Another mechanic told me its due to a compression misfire (if that makes sense, I don't think that's the literal term) and that I need a valve job.


Thank You again for your feedback, it IS helping narrow down the problem.


Thank you so much.

<Okay I tried revving it in neutral and park, the rev limiter stops it at 4k rpm and no misfire,>

What do you mean, NO misfire??
That is what the rev limit does..it drops the injectors out at 4k, causing rev limit.

The rev limiter prevents the car from going to higher rpm, in which causes the misfire.

So in N and P it doesn't go to a high enough rpm to cause the misfire.
But in 2nd and 3rd it misfires.

My question is "Does it misfire at 4k in each gear?? "
I need that EXACT info

Yah regardless of what gear its in it will misfire.