1986LPE383

Problem 1. Surging problems on cold start on 1986 with a 383 engine, 58mm TB, super-ram and a LPE 219 cam. Was okay for a while. Now car will not start without opening the throttle blades. Car then surges between 1200 rpms and 500 rpms (with steady throttle opening) for about 10 seconds then settles at 900 rpms (ecm idle set at 900 rpms for this cam). Car should start without the throttle blades open. TPS at .45v (.01v low). Replaced the IAC. Same problem.

Problem 2. Tried replacing the 58mm with a 48mm rebuilt TB. Same problem as above except the idle speed settles at 500 rpms (not 900 rpms as required for the cam). TPS at .50v (in spec). Swapped IAC’s with no difference.

BTW, reset both IAC’s per the book (128mm setting, key on-engine off for 60 sec with AB connected at the ALDL, unplug IAC connector, run engine till hot, reconnect IAC, disconnect battery to reset ecm, etc., etc.) IAC air passages are clear. Vacuum hoses (3) are connected.

The 48mm is going to be installed on the car as the 58mm causes surging cruise control problems at highway speeds. The 48mm doesn't.

Any ideas as to the surging problem & the 48mm not setting at 900 rpms?

vader86

Have you tried setting the TPS to 0.54 V?

leesvet

with all those changes, I would think you had the "tune" modified to manage the cam and intake.
have you? had a new "tune" burned in a prom?

There is no need for a 58mm TB. That's aftermarket sale BS at its finest. The 48 has more than enough air flow to feed that engine...even if it were turbo'd. The 58 is screwing with the intake vac and THAT is what is causing your CC to act up. With the cam, vac is probably down anyway.

383vett

The 58mm should be fine for that motor. I ran 58nmm on my 383 with a SuperRam and a more radical cam and had no idle issues. Have you played with your minimum throttle blade opening adjustment.

bjankuski

I agree, the throttle needs to be set for a minimum idle speed that you are going to see with your combination. If you are never going to idle less then (lets say) 700 RPM when the engine is warm and at idle, you would like the throttle blades to be opened far enough to have the engine idle at 700 RPM with an IAC count of around 0 to 10. This will help stabilize the idle and allow the headroom for the IAC to open up and control the idle when the engine is cold. With a modified set-up the factory minimum idle speed settings of 450 RPM should not be used.

You may also have a timing issue or open loop fuel issue that is causing the idle to surge when you start the car. Superrams are picky on the fueling and timing settings at idle, if they are off the car will surge.

As far as the cruise control surge is concerned you can solve the problem by remounting the cruise control cable farther from the center of the throttle shaft. Since the throttle open farther with any throttle movement cruise control needs to be slowed down so it does not effect the car as much for its given movement. Make a bracket the repositions the cruise control cable farther from the center of the throttle, the problem will be eliminated.

91blkvette

Sounds like a fuel map problem. Having the same issue with my Megasquirt controlled big cam 427 (same intake and TB too). Have to give it throttle ( = more air) to get it started, idles high too, then settles.

One reason provided was too much fuel commanded. Another is a vacuum leak.

Vett1990

Lets keep this post going.... I have almost the same issue with my 90 Vett 383, it ran great the first summer then went to sh_t. With your '86 I would assume its MAF where mine is MAP or density air. If you mess with the min idle it also effects the TPS reading, my car will not idle at a stop light its the old one brake and one the gas routine. I plan on taking it to the dyno this winter to get it re-tuned under a load. One thing a have noticed, my ECU becomes very warm almost hot to the touch. Is that normal or.....?

Keep us posted on all findings.

91blkvette

Updates? Having the same problem with a Stealth Ram on a big cam 427.

blackozvet

other things to check,

Coolant temp sensor is working ok,

Cold start sensor (if fitted with one) is ok,

Fuel pressure - not too high and vacuum line fitted for street use.

91blkvette

Those of you using a 58mm TB, can you look at where my butterflies are and compare to where yours are? Set screw is probably close to full travel.


The only way to get my engine to idle is to open the flies way up. I'm using a MS3 and I see my IAC counts moving..but adjusting fuel either makes it stumble or lean it out.

Guessing it's due to the cam, making for a challenging tune ahead.

bjankuski

That seems like a lot of throttle opening for a 383 with the 219/219 cam. That is a small cam that should not require that much throttle opening. Is your base timing set correctly at 6 degrees with the timing connector disconnected?

Vett1990

This may sound crazy but, I found the issue with my 383 idle problem. First off, I use my 383 as intended... always testing it full potential. i spent some time this past winter changing the transmission and cleaning up a few minor leaks.

My valve covers were dripping on my headers so I thought it would be best to replace the gaskets, no problem you say. What I found under the valve cover also fixed my idle issue... Issue being the #8 intake valve spring was broke and had been for nearly one northern driving season, please thank the big guy upstairs for not allowing the second spring to brake on my Dart Pro 1 head.

I replaced the spring, the engine fired instantly with no need to apply any throttle, and idled back at the 700-800 rpm, no stalling when put in gear.

91blkvette

I have a 427..and a bigger cam than the 219.

Agreed, it seems too much for me as well. Having issues trying to tune the MS3 as changing pulsewidths don't seem to help settle the idle. Can't start it without adding throttle, and holding a bit of throttle it will idle around 1000 and with an AFR of just under 12 or so. Almost confirmed my base timing at 10, but the engine shut off just before I could visually confirm the reading on the damper..

Will be trying again tomorrow to get a "no foot" idle..

383vett

My 406 has a 236/246 cam with a 58mm tb. I just checked and it's blades are opened about 1/2 as much as yours.

91blkvette

That's what I was hoping NOT to read. Well..back to square 2 and work on pulse widths.. Thanks for the info though.

91blkvette

Finally got this beast to idle at 1000-1100..but I still have to open up the butterflies, and add a bit of right foot. Tried closing them, and could not get the engine to fire. Changing MS3 pulse widths only make it stumble..

But...the sound of a 427 with only muffler eliminators..is awesome..