I'm having trouble with the idle on my 1990 Gen I 3.0 EFI (manual trans). Basically it's too high: starts out around 2000 when I first start it. Drops down to around 1250 once it's warm, so it seems like it's running about 500 RPM high.

The deal on the vehicle: I just rebuilt the engine. So it's mostly all new stuff, i.e. spark plugs, distributor cap, fuel filter, etc. I also replaced the ISC with one of the new ones from DSMISC, so I believe that it should be good.

The FSM only lists three things to check in their troubleshooting matrix for "Incorrect Idle Speed (when cold)," they are: coolant temperature sensor, idle position sensor and ISC. I've checked and rechecked the first two and already replaced the last. Any suggestions as to what else might be causing the problem?

I has a previous saga on this forum about trying to get over major hesitation on this truck. I ended up setting the timing, but could never do it the "official" way because, for some reason the truck won't start/run when I ground the ignition timing connector. So I set it by feel (around -5 BTDC without grounding the connector); the truck seems to run pretty well (no hesitation, plenty of power) but the high idle is really annoying.

Is it possible that the ECU is causing the problem? Any way to diagnose this? Also, the FSM gives instructions for setting the factory idle speed; but I really only want to do that if it seems clear that this is the place where it needs to be adjusted.

any suggestions would be appreciated!

thanks,
steve

i am in a similar boat.

just rebuilt the engine. starts up around 2000+ rpms and works itself down to under 1000 rpm after a few minutes. i got a little stamer at 1st but like you when it runs it seems to run fine. i hope someone got something.

the ISC is adjustable small screw and a allen wrench on the outside of the isc housing

I have a similar problem...more pronounced in cold weather. I am also abbreviation illiterate. What does ISC stand for?

Thanks for the info.

Idle Speed Control


John B.

There are two idle control systems on the 3.0.

The most known one is the ISC circuit, a stepper motor with a sealing plunger that modifies an orfice opening in the TB, controlled by the ecu. The second FIdle system is a mechanical wax pellet capsule type that controls the first few minutes of idle, raising idle speed on first cold startup for emissions control, and it can only be fixed with a new TB. I think that one is ok on yours, because you get a higher first start idle that comes right down.

I suspect one of two things.

First, somebody has messed with the idle stop screw on the main butterfly to "fix" and idle problem caused by a bad ISC in the past, raising the mechanical base idle speed, in which case you have to do the fsm base idle speed setting they say is not necessary (so why do they tell you how?). This is fairly complicated, and also requires resetting the TPS/IPS so it reads proper throttle idle position and open position.

Second, you have a vacuum leak beyond the capability of the ISC to compensate. Look for it by sound on an idling engine using a piece of vac hose in your ear. You can hear the hiss easy that way, and localize it. Scan the end of the hose around (it's very directional) and you'll home in on the source.

There is also an air bypass screw on the top of the TB, perhaps under a cover, down in a hole. Try adjusting that to curb idle speed after warm up. Since that's the easiest, I might try it first.

Eddy when are you coming to NC to help check this stuff out? <img src="/forums/images/graemlins/smile.gif" alt="" />

I go to WNC quite often, riding in the woods, but the part you have to go thru SC to get to is a long haul away for me, and every time I go to SC I get in trouble...

So I have fiddled with the air bypass screw; I've tightened it as much as possible to get the idle as low as it will allow, so I don't think there's any more to be done there.

On the vacuum leak question, where exactly should I be looking? There aren't very many vacuum hoses on this thing (brake booster, fuel pressure regulator and evap canister, from what I can tell) and I've replaced those except for the brake booster. I assume that any leak of interest would be downstream from the throttle body? The only other places I can think of would be the gaskets, i.e. TB <-> plenum <-> intake manifold <-> heads. Those are all new too, of course (the gaskets).

I'm saving the adjustment of the base idle for last. I can't swear that that screw has never been tampered with, but I know that I've never messed with it.

thanks,
steve

I would start at the tb and check it all for vac leaks. The hose in ear method will find all but the smallest if you can probe everywhere, which is not easy. There is the tb/water plate joint, the water plate/plenum joint that are gasketed, the isc oring, the various vac lines, the plenum casting itself, the upper/lower plenum joint, and the lower plenum/heads pair of joints.

One way to check for vac leaks in individual intake runners (between the plenum and the intake valve) is a vac gauge plumbed to idle vac, not ported vac. Watch the gauge at idle. If it ticks down once every two engine revs, you have a leaking runner or some other single cylinder sealing problem. A vac gauge is a dandy and very cheap diagnostic tool. Google using one for an education.

The wax pellet fidle valve is also a suspect, despite what I said earlier. If it failed open, you have a good sized air path right there, and the isc can't cope.