'94 SC1 idles at very high RPMs

[ivkowalenko]

I've just gotten my SC1, and its top three problems is a leaking valve cover gasket (easy to fix, we can handle that), a problem in the cooling system (previous owner says it's a cracked radiator, gave us a new one, but I don't see any leaks; maybe they plugged it with a stop leak compound and now it's causing trouble in the whole radiator? We'll figure that out when we replace the radiator) and the car idles at high RPMs. The Idling issue is one of the ones that concerns me the most, because we're not 100% sure what's causing it, it's causing the car to gulp down fuel, it's certainly heating up the engine (exacerbating the cooling issue), and it's making it a challenge to drive the car slowly (If I let go of both the clutch and the accelerator, the car typically continues to accelerate, but at a much slower pace).

At first we thought the problem was that the accelerator or throttle needed adjusting, but having driven the car around a little bit, I'm not so convinced.

When I start the car, it idles high. Typically between 2,000 and 2,500 RPMs. If I start driving and press on the clutch (to shift gears), it shoots up in the neighborhood of 3,000 RPMs (±250 RPMs) and stays there for a good long time. I can coast for about five minutes before it even starts to drift down.

The car is a 1994 SC1, with the LK0 engine (SOHC), and a 5-speed manual transmission. I am technically the fourth owner, but functionally I am the third (the person I bought it from never drove it, and let it sit for about five or so months in their driveway before I bought it from them). The last owner to actually drive this car got it from a co-worker. When he got it, the car didn't run, but after "some tinkering" (that's what they told me, and the wife of the guy couldn't be more specific) he got it running. We also know that it starts harder in the winter, but it starts eventually, every time. I don't know if any of that is related, but I figure more information can't hurt.

My theory (granted, I have little experience with fuel injected engines, so I could be way off on this one) is that something is causing the throttle or the injectors to stick open. The theory of my more mechanically inclined family friends is that the (functionally) previous owner may not have connected the vacuum hoses correctly, and that's causing an issue. I'm just looking for any feedback that the fine people around here might have with this.

Thanks for any information you can give me on this one.

[Baron5867]

Originally Posted by ivkowalenko

My theory (granted, I have little experience with fuel injected engines, so I could be way off on this one) is that something is causing the throttle or the injectors to stick open. The theory of my more mechanically inclined family friends is that the (functionally) previous owner may not have connected the vacuum hoses correctly, and that's causing an issue. I'm just looking for any feedback that the fine people around here might have with this..

Sounds like a vacuum leak. Check all hoses, there should be a vacuum diagram on the front of the car to verify connections, if not there is probably one online. Even if connected properly, make sure the hoses themselves are in good shape, with no leaks. Another possibility can be a malfunctioning IAC (Idle AIr control) valve. Finally, you can check for any leaks at the throttle body (pretty sure you have a TBI) and the intake manifold.

[ivkowalenko]

Originally Posted by Baron5867

Sounds like a vacuum leak. Check all hoses, there should be a vacuum diagram on the front of the car to verify connections, if not there is probably one online. Even if connected properly, make sure the hoses themselves are in good shape, with no leaks. Another possibility can be a malfunctioning IAC (Idle AIr control) valve. Finally, you can check for any leaks at the throttle body (pretty sure you have a TBI) and the intake manifold.

So, this past weekend we finally got to look at it. Sure enough, the hose going to the PCV valve was pretty much falling apart, and a couple smaller elbow joints near the TBI were in bad shape. The PCV valve was also in bad shape, pretty much useless. Replaced them all, now the engine will (in some situations) idle down around 900-1200 RPMs. TBI is in good shape, so no problems there (we think).

The engine seems to be slow to idle, though, while driving. When I engage the clutch, it takes a second or two before the RPMs come down, and then it's usually to 1.2-1.5 KRPMs. If I give it five or more seconds, it'll come down to 900 RPMs again. Not entirely sure what this is. Perhaps a bad TPS sensor? Or could this also be the IAC? I know the computer is throwing code 23 (I'm going to double check and make sure I didn't miss any other codes), but would a bad air temperature sensor in the air intake passage of the air induction system cause any kinds of problems like these (I mean is the computer trying to "guess" what it should be doing based on this information?)?

[madpogue]

RPM should drop when you DISengage (push in) the clutch. But it should only drip to about 1300 RPM while the car is moving. It then should drop to 800-ish when you come to a stop. If it's hanging above that for a period even when stopped, then it might be a bad TPS, or IAC.

[fdryer]

Two major issues to deal with surrounding an old S-series car; the faulty original round nosed plastic coolant sensor and thermostat. Replace both; a flat nosed brass sensor and a 195F t-stat. You have two coolant sensors; a single wire one for the temperature gauge and the two-wire one that fails on every old S-series engine. Sensor and t-stat conspire to hold idle above 1k rpm (vacuum leaks aside). The air temperature sensor is fine as it sits in the breeze measuring air temps. Assuming no one meddled with the factory set throttle stop screw, blocking off the idle air bypass hole (in front of the throttle plate) should have the idle drop down to 500-600 rpm) to check the throttle stop screw/throttle plate position. A higher idle would suggest either someone meddled with the stop screw or a vacuum leak is keeping the rpm up (intake manifold gasket?).

A well maintained engine with minimal maintenance will always start immediately, idle high (1200rpm+) while warming up, gradually idle down in direct proportion to coolant temps until a normal idle (800-900rpm) is reached in less than 10-miles of driving.

[alordofchaos]

Originally Posted by fdryer

the faulty original round nosed plastic coolant sensor

+1, it costs nothing to check it and only $10 to replace

[ivkowalenko]

Originally Posted by madpogue

RPM should drop when you DISengage (push in) the clutch. But it should only drip to about 1300 RPM while the car is moving. It then should drop to 800-ish when you come to a stop. If it's hanging above that for a period even when stopped, then it might be a bad TPS, or IAC.

Sorry, I used the wrong verbiage. I always get that backwards, even though I have a vague understanding of how it works, mechanically speaking. About 1,300 RPMs while moving sounds right for my situation. However, as soon as I disengage the clutch, the RPMs either go up a couple hundred for a second, or stay right where they were at. Is that normal?

Originally Posted by fdryer

Two major issues to deal with surrounding an old S-series car; the faulty original round nosed plastic coolant sensor and thermostat. Replace both; a flat nosed brass sensor and a 195F t-stat. You have two coolant sensors; a single wire one for the temperature gauge and the two-wire one that fails on every old S-series engine. Sensor and t-stat conspire to hold idle above 1k rpm (vacuum leaks aside). The air temperature sensor is fine as it sits in the breeze measuring air temps.

I'll check those out. We may handle that the same time we replace the radiator (we're told it is cracked, but we see no evidence of leaks, and the temperature only gets up high if the car isn't moving for a prolonged period of time).

Originally Posted by fdryer

Assuming no one meddled with the factory set throttle stop screw, blocking off the idle air bypass hole (in front of the throttle plate) should have the idle drop down to 500-600 rpm) to check the throttle stop screw/throttle plate position. A higher idle would suggest either someone meddled with the stop screw or a vacuum leak is keeping the rpm up (intake manifold gasket?).

It doesn't look like there's a throttle stop screw, it looks like a block of metal. The car does get down to the neighborhood of 900 RPMs, so it may not be modified.

Originally Posted by fdryer

A well maintained engine with minimal maintenance will always start immediately, idle high (1200rpm+) while warming up, gradually idle down in direct proportion to coolant temps until a normal idle (800-900rpm) is reached in less than 10-miles of driving.

This sounds right, except it hits that 900 RPM range before ten minutes. Usually I let it sit for a few seconds before I start off, and it'll hit 900 RPMs soon.