okinawa86vette

I've been fighting this problem since I put the new motor in. It will fire up, then die. If I give it a little gas it'll stay running, or if I adjust the min air screw to give it more air. Once running it hunts for the idle and eventually settles down. The car's an automatic. When I put it in gear, it kills the motor unless I give it a little gas just as I put it in reverse or drive. I thought I had the problem solved a week ago, when I put a new(not reman) maf and relays in. It ran great, but yesterday, when I went to start it up, it ran much like it did before. The idle was where it's set in the chip, 850 in park and 750 in drive.

For a while, I was getting both maf voltage high, and voltage low codes, as well as a EST code 42. When I replaced the maf, I only got the code 42. I replaced the ignition module after following the t/s chart in the shop manual and it's code hasn't come back.

It was running very, very rich after I disconnected the battery to reset the computer, but after I helped it idle for a bit and ran it through the rpms, it settled down, once the blm learn started. Bare in mind, I'm at over 6500ft, so the car running rich after an ecm reset is normal.

There are absolutely no vac leaks. I had a shop do their smoke test when it was finished and it came out great. I also did the home mechanic test with carb cleaner to see and the idle never changed.

Wiring and connectors all look fine, no corrosion or exposed wires.

Fuel pressure is set to 43psi, and with the car running with and without a load, it holds without dipping.

Base timing is set to 6* btdc

What I have replaced to date:
-Maf....twice. When it started acting up again, i replaced the maf under warranty with another new one and it still acted the same.
-both maf relays
-ignition module
-02 sensor, even though it was giving good readings on the scan tool and was toggling rich and lean, i figured it couldn't hurt to try it.
-IAC
-ignition module

I have played with the min air screw to get it to settle down, but it'll either start and idle ok when its set opened up more, but still die going into gear, or when it's more closed, die durring startup and occasionally let me go into gear.

BLM value is 118

IAC is 145 when the car is at it's happiest, which I know seems high, but when I open the min air screw to drop the IAC counts closer to 30, it idles way to high and still won't go into gear.

TPS voltages look good, ranging from .54 with no throttle to 4.15 at
WOT

My major setup items are:

88 coupe, automatic.
GM Perf 383 decked to give 10:1 compression
Edelbrock E-tec 200cc heads
LT4 hotcam
1.6 ratio rockers
SDPC baseplate
25lb svo injectors
super ram
58mm throttle body
MSD 6AL
ed wright custom tune

Right now, the car won't run right, but no codes are being set. I almost wish it would, to give me a direction to go in.

I'm lost. I can't think of anything else it could be. I'm open to suggestions.

BTW, it runs strong. Despite the finicky nature, once it's in gear it runs great. I ran a 13.5@101.8mph in Pueblo. Track alt is 5000ft and using the nhra conversions, it comes out to mid 12's at 106-108 mph. I know 13.5 seems slow, but to put it into perspective, most of the Z06's I've seen run high 13's out here, WS6 LS1 f-bodies run 14.7's and my viper driving friend, with a few perf mods, runs 13.4's.

Thanks, guys.

Chris

KyRP2NITe Corvette

Have you looked at the idle air control motor? Sounds like the car runs fine while using the pedal right? Well with out the pedal the Idle air control motor (it's really a valve) controls the engine speed... the hunting could be due to a faulty or fouled IAC....the rich condition is probably more that the ECM needs to re-learn the optimum settings for your alt. and a little more driving may clear that up..but the hunting at idle and stalling at idle when placed in drive sounds like a a faulty IAC to me.

Morley

since you say you replaced the IAC, did you clean out the passages in the TB when you did this?
Also, when looking for vacuum leaks did you check the throttle blade shaft? They are known to leak air as they get older and sloppy.

Pete K

I had this exact problem. In my case, it was the weatherpack connector at the iac motor. Bad wire at the spot where the crimp was made. Very hard to find but easy fix once I did.

bogus

I am with Pete. You see, the pre-91 Vette's used wire that has a tendency to wear and break. The insulation fails over time and things just get ugly.

I would start with checking each and every wire on the MAF and see what you learn.

Also, check the harness on the drivers side of the engine. You may have bad wiring inside the harness and that's a real pain to fix.

okinawa86vette

Ok, to answer some.....

The IAC was cleaned, along with the passages. I also tried a new one, and it seemed to react slower and run worse. The scan tool shows that it's doing more or less what it's supposed to, but the motor's rpm hunts faster than it can keep up, I think.

I checked the wiring for the tps, iac and maf, but the iac connector could still be suspect. I seem to be getting ok iac readings, though, and it reacts to changes.

The car runs ok when I'm on the gas and even idles reasonably when I'm in gear. It's just when it's in neutral/park that it's at its worse.

Thanks for the suggestions. I have to wonder if it's a bad ecm. Everything else was checked. I'll t/s the iac a bit more tomorrow.

Pete K

If you haven't done it already, disassemble the ground cluster at the side of the block, near the oil filter. Sand them and reinstall using the star washer underneath the wires at the block. I have seen similar problems on an f-body. Drove us nuts. Spitshining the grounds fixed us up. Something to try that cost nothing but some time.

ZD1

We have (actually had) the exact same setup. There's a couple of thing to keep in mind

1) the ed wright chip is not even close to what you need. It's tough biut look at getting an innovative WB-O2 meter, tuner-cat s/w, datamaster s/w & cable...then email me.

2) the stall on your converter is to low...I moved to a edge-reacing 3200 which for whatever reason was to0 high...my current 2400 is tunalble but will require a good balance of ignition timing and RPM

3) You'll never be able to adjust idle "stuff" messing with the TB, ignition, fuel pressure, etc... SO STOP IT!

have a nice day...hopefully I can save you the Kilo-dollars I've spent

okinawa86vette

I have a hard time believing the car won't idle correctly in park because of the torque converter.

All of the things I've been working with, fuel pressure, timing, ignition, throttle body, etc, have a direct effect on idle quality. If the fuel pressure is erratic, the timing off, the ignition malfunctioning, etc, the idle will be affected.

I don't think I'm far off the mark when it comes to these things. The motor does the same things with the stock chip as well, but runs much, much worse, so I don't think it's an issue there.

Chris

tjwong

Chris, have you tried to bump the base timing up? At your altitude you can run more timing without knock. With a cam change you need to raise the cylinder pressure back up at the low RPMs. And one way is to give it more timing. Also you mentioned code 42, is that present now? If it is you need to find the cause for that. If an EST code sets, the ECM will retard the timing, which then can cause your idle hunt. Depending on the ECM strategy I have seen as much as 12 to 15 degrees of retard.

I normally add timing in the closed throttle tables in the chip. This helps to stabilize the idle in these cars. But you can also do it manually by bumping the base timing up.

K87ZZ4

As the early responses suggested, I think it's likely the IAC. I had very similar behavior with mine... start and immediately die. It was an intermittent problem. I thought it was the MAF, and repalced that, but in the end it turned out to be the IAC. I took the TB completely apart and cleaned it well, put in a new IAC and it's been fine since.

The tough part was that it was not setting any codes... the ECM was telling the IAC what to do, the IAC just wasn't doing it (sometimes, of course). I think the only feedback to the ECM is if the IAC fails electrically. My guess is that the IAC was sticking and it wouldn't open far enough to let enough air pass for the engine to idle. I could get it to start and run by giving it part throttle.

I know it's frustrating--I spent a LOT of time trying to diagnose the problem--and wasted money replacing the MAF when I didn't need to. In the end, I still had to guess that it was the IAC. Best of luck.

okinawa86vette

Well, guys, thank you for all the suggestions. I guess it went back to basics on this one. I went through all the t/s charts, ohmed out all the involved wires and the iac seemed to be working perfectly.

Just for kicks, I decided to check the plug wires and make sure I didn't swap a couple wires along the way. I found some knicks in the number 4 wire. I unplugged it, and started the car. It ran exactly the same as it had been. Then I plugged it back in, and unplugged another, and it ran worse.

I replaced the wires with a new set, and it seems to run well. The ecm's still learning everything, and I only took her for a short drive. There was a good puff of black smoke the first time I went wot, which i imagine is the unburnt fuel from the #4 plug not firing and it running rich from the computer resets. It bogged a little, but after the 10 min drive, it got better and better. It seems to be getting closer and closer to where it should be. It pulls much harder.

I can't wait to see what she does at the track now that she's running on all 8 cylinders.

I'll keep you guys updated on how it all turns out.

Next car I buy is only gonna have some plug and battery wires under the hood, so the computer stuff doesn't cloud the problem.


Chris

bogus

Interesting.

I agree with tjwong. I missed the detail you are running an LT4 hotcam... that needs more like 12-15 degrees of timing to tune correctly, if I remember rightly.

okinawa86vette

You guys think I should run 12* base timing?? That seems like an aweful lot, considering I tried setting it to 10 and it bogged and knocked horribly.

Morley

To do it right you'd need to reprogram the prom to reflect the new 12* base..otherwise it thinks it is still 6* and your advance in the tables will take it beyond the max they are programmed for.

okinawa86vette

Alright, then, that'll have to wait. I'm gonna actually try to drive it for a change. I'm sick of t/s for now.

Thanks for the advice and help, guys.

Chris

KyRP2NITe Corvette

True Story:
Friend of mine had 95 Camaro LT1, he installed a hotcam himself but accidently installed it 180* out...that is he lined up the dots so that they where both at the top (12 o-clock) postion instead of facing each other (12 and 6 o-clock positions). That car started and ran! Felt fine (but under powered) while driving around but idled like trash. I am still baffled by how this worked but after lots and lots of T/S he finally decided to pull the cam and look for a wiped lobe, we got as far as the timing chain cover being removed and I noticed that something didn't look right ...bam there it was.
Moral of the story, not that you didn't double check everything when you installed the new cam but is it possible that you did the same thing??

ZD1

I'm running 14 degrees

okinawa86vette

The motor was built by the race shop at Scoggin Dickey, then installed by a local shop, and I had them double check all that kind of stuff, including torque specs, the cam, etc, before they installed the motor. I did the last motor myself, but my back had gotten bad enough I couldn't do the work myself this time around.

I really don't think the car was down on power, considering it was out running Z06's and the number 4 cylinder wasn't even firing. Now that I've got it more or less sorted out, I need to get in some more runs.

I appreciate the suggestion though, it had been thought of in the "WTF is wrong with this thing" chain of events.

I also played with the throttlebody min air screw setting. I had to open it way up, and now I've got the IAC counts around 30 or so and I can put it into gear and it doesn't even come close to stalling. It still idles a few hundred RPM high, but I haven't driven it around much to let everything settle in, as that always seems to help. I also swapped out the IAC with a known good one from my pickup. I had a spare when I was t/s a high idle in the truck, but it turned out to be something else. It seemed to react much faster than the old IAC, looking at the scan tool.

I don't even want to tally up the total parts count right now. I'm just glad it's driveable. It pulls much harder, especially after the plug wire change(now that they all work) and ignition module.

Also, with 12* base timing, I actually get less knock counts than I did with 6*. I may play with it and bump it up to 14, but the SDPC guys recommended 8-12*.

Thanks for the suggestions, guys. I'll post a time when I get it to the track next.