M20 Stroker - High Idle

[Otto Smyth]

Howdy gents. First post from my new account. It’s been a while since I’ve logged on. Used to have a shop account for my shop in Dublin CA.

I’ve got a car that I’m working on: 1985 eta car swapped to an I drivetrain. Engine was built as a 2.7l stroker. Ross pistons, stock cam. That’s about all I know.

It has a Squid tune to match the mods and runs fine. Trouble is, it idles at 1100rpm hot, all the time. The customer swapped 3 idle valves, air flow meter, DME and who know what else.

I’ve confirmed cam timing and verified there aren’t any air leaks.

Any ideas? The idle valve doesn’t do anything if you unplug it, but it has signal and if you open the shutter with a screw driver it snaps shut when you plug it in (key on, engine off).

[ForcedFirebird]

Is there evidence the throttle stop screw has been tampered with? Vacuum leaks generally cause a rolling idle on AFM/MAF cars, MAP/SD/Carb systems are the ones that high idle. If the screw was tampered with, be sure to re-adjust the throttle switch so the ECU "sees" zero and wide open throttle.

[Otto Smyth]

Thanks for the reply. Sorry, I forgot to mention. I set the throttle plate to spec and confirmed throttle position switch function and adjustment.

[digger]

if the idle is high there are only so many things that cause plausibly be
- too much air getting in (a vacuum leak, throttle blade opening too large, ICV position too far open)
- ignition timing is too high

disable the ICV and plug all the holes associated holes and what happens?

[Otto Smyth]

Digger, I couldn't agree with you more. I've been working on E30's since the 90's. This is a first for me. When he originally called I was like "yeah, sure bring it over." I thought I'd be finished by now. It had a Nuke Performance cam gear set to zero. It didnt allow the cup behind the rotor and was rubbing on the front cam seal plate bolts. I shitcanned that and went back to stock. I smoke checked it for leaks. If I induce an intake leak the idle drops. I was able to set fuel mixture to .5V at the O2 and it hunts up/down normally.

I havent bypassed the idle valve yet, but the customer said that he had and the idle dropped to normal. He has multiple idle valves from unknown sources. My next action is to grab a known good valve from a running car and try that.

it's kind of a tricky problem to get involved with. I didn't have a baseline to compare it to since he never had a stock, running engine, in the car. He built the stroker and did a junkyard "i" swap with bits and pieces he found.

If I remember right, the idle should hunt up and down pretty bad if you unplug the idle valve. Can anybody confirm that? I could be thinking of the earlier cars, though....

The only other kinda goofy thing is that the brake booster cyclone valve setup is missing. He plugged one hole on the throttle body and is just using a single check valve. I don't see why that should cause an issue, though...

I guess I should unhook the brake booster and plug it to make sure it isn't leaking. The smoke test should have shown that, though.

I'll try bypassing the idle valve today. I may need reference coolant temp sensor values, too.

Trying to get my brain around this one. Thanks, guys.

[Otto Smyth]

Any chance anybody out there in R3V land could unplug their idle valve on a “I” car and tell me what it does? I’m 99% sure that I should get hard surges up/down about 1k RPM’s.

[Otto Smyth]

FWIW.... brake booster is fine. No air leaks there.

if I remove the idle valve and plug the air boot and throttle body fitting it idles at about 800 steady.

Idle valve is some strange aftermarket rebuild or something. I’m going to grab a Bosch valve to try when I can run across town.

[ForcedFirebird]

No, unplugging the idle valve will just make the engine continue at whatever value the valve was last at. For instance, when young and an e30 had a vac leak (rolling idle), I would unplug the valve just at the moment when the engine was at 800 and it would just stay there. Rolling idle happens when the ECU is "trying" to compensate an air leak between the AFM and the intake valves. On any e30, you can unplug the evap line from under the TB, loosen the oil cap, or pull the dipstick and the car will roll idle, not go into high idle...that is why I find your situation so strange.

Have you tried a smoke test or at least using some brake/carb cleaner in/around the intake manifold/TB areas? That will usually be a dead giveaway. I once had a weird idle on a customer car, and it turned out that one of the manifold gaskets rotated while he was installing the manifold, so that only one gasket's hole was on a stud (gasket basically hanging half way out). Took a smoke test to fond that one.

[Otto Smyth]

No, unplugging the idle valve will just make the engine continue at whatever value the valve was last at. For instance, when young and an e30 had a vac leak (rolling idle), I would unplug the valve just at the moment when the engine was at 800 and it would just stay there. Rolling idle happens when the ECU is "trying" to compensate an air leak between the AFM and the intake valves. On any e30, you can unplug the evap line from under the TB, loosen the oil cap, or pull the dipstick and the car will roll idle, not go into high idle...that is why I find your situation so strange.

Have you tried a smoke test or at least using some brake/carb cleaner in/around the intake manifold/TB areas? That will usually be a dead giveaway. I once had a weird idle on a customer car, and it turned out that one of the manifold gaskets rotated while he was installing the manifold, so that only one gasket's hole was on a stud (gasket basically hanging half way out). Took a smoke test to fond that one.

I have smoked it. No leaks after I fixed the fittings on the throttle body.

[Otto Smyth]

This one is solved. Aftermarket idle valve. Not sure where it came from, but it looked just like a Bosch. Swapped a real Bosch valve in and it's good to go. Thanks for playing, guys!!