Well........here's my MY FIRST POST

Let me start from the beginning. Car is a 85 IROC. The engine is "supposed" to be a 383 stroker, but I have no way to verify, except for the butt dyno, and when it's running right, it is a MONSTER, even with stock manifolds. Anyway, I KNOW the engine is a '87 L98 from a F-Body from the Suffix codes. So we'll go with that.

Stock 85 TPI system in place, everything is present AFAIK except for the AIR pump which was MIA when I got the car.

SYMPTOM : When I first got the car, it would spark knock. If you manually shifted through the gears the car pulled like a horse all the way to redline....if you're tooling along in 3rd or 4th ( 700R4 ) and you go to say pass someone, when you give it about 1/2 throttle or a little less, you get spark knock, AUDIBLE. AS SOON as this happened, car would set a code 42, which point to the ESC system...

So the 1st thing I did was find out that the IROC had a Hypercrap chip installed. Luckily I found the box, and in the box was the original chip. I swapped chips, and reset the ECM.

Got in, no other changes, car drove GOOD!!! Made SEVERAL WOT runs and it was awesome......no spark knock.

Then later that day, it started it again........

So I started with the basics, as I planned to anyway since I just bought the car...

- Spark plugs
- Spark plug wires ( looked like the friggin' original GM wires )
- Distributor Cap and rotor ( they were pretty nasty )
- Fuel Filter

Replaced all these......all needed to be replaced regardless. Went for a drive, notice much better idle ( car does have a bigger than stock cam, definite lope through the exhaust, but not too bad ) part-throttle better.

Tried to duplicate the spark knock, and it DID IT, just the same. Except now, when you floor it quickly, it would stutter and backfire.......never did that before. I went back and verified all plug wires are correct and seated on plugs and dist cap.

Also, WINALDL displayed that the ECM was setting a code 42 ( EST ) and a code 32 EGR.........


SO next......

I set initial timing ( EST bypassed ) to 6 degrees BTDC.

TEST DRIVE.........same thing........both backfire / stutter and spark knock.

I set the minimum air adjustment ( IAC ) the correct way, making sure everything was right. Then I ended up adjusting the TPS, as it was at .7v at closed throttle. I bumped it down to .6v per WINALDL readings.

So now I'm thinking I could have two seperate problems, and was looking towards the knock sensor for the spark knock issue.......since it still sets a code 42 when you get it to spark knock, and will sometimes set it when there is no AUDIBLE knock.

But first I wanted to verify the EGR wasn't the problem. So I unplugged the EGR vacuum solenoid.......I followed one of the vacuum lines to the EGR valve itself.......couldn't make out where the other one went, and to be honest I kind of forgot about..yeah I know...I'm an !#$%

Anyway, I moved on to other things, notrealizing I had a vacuum leak from the other EGR vacuum line ever since I disconnected and removed the EGR solenoid......

So, since my fuel pump relay was coming apart ( literally ) I replaced it, and also replaced the knock sensor with a unit for a '87 TPI 350.

Reset ECM, test drive.......

NO spark knock.....AT FIRST.......after a bit of driving, I duplicated it again......it set a code ( I'm guessing 42 ) then the SES light went off again.......

Backfired like a big dog!!! I tried to floor it from a slow rolling start and it backfired loud enough to think somebody was shooting.......

let off, got back into it gradually, all the way to redline, and it acted fine.

At this point, I still hadn't realized I'd caused a bad vacuum leak from the EGR line being left unplugged underneath the plenum...

So I figured I mine as well try to rule out the ignition, so I installed a new ignition module, the one in it was a original looking GM unit, with non of the dielectric grease or anything installed on it......

TEST DRIVE............same stuff.......big backfire again and spark knock on tip in from high gear......funny thing is, it's idling in gear and Park better now than it ever has after my IAC and TPS adjustments...

This is where I stopped, as I'm leaving for a week's vacation tomorrow morning with my family......but I'm off the following week as well, so I plan on working on the car then.....

I FINALLY figured out about the EGR vacuum line......tried to grab it from under the plenum so I could plug it or melt it shut for now, and of course it came out of the fitting under the throttle body / plenum area......


SO............looks like I'm going to pull the plenum etc next week. I think I have an intake leak anyway, and I'm sure the vacuum lines under there could use replacing, as well as testing the EGR valve, cleaning the throttle body and plenum off the car as well.


Any ideas on what might be causing my troubles?

Anyone know if any other GM cars used the '870 ECM? This info would be in the F-Body parts manual, and could tell what cars I could hit up for a spare '870 ECM to try at the junkyard.......

Any help, suggestions, tips, comments etc greatly appreciated.....

I'm looking towards the fuel pump, as I need to verify fuel pressure when I get back, and after that I need to test the ESC module.......

Thanks
Chris
85 IROC "Problem child"

 

If not mistaken, the "870" was only on the 1985 models. You said in the write-up that engine has a big cam. Try adjusting the timing down to about 3 to 4 degrees and check fuel pressure. Should be around 45 PSI