1988 3.5 v8 EFI High idle rpm

My idle rpm has started sitting around 1500.
Removed and cleaned the Plenum and double checked the seals are good.
Checked for air leaks at all hoses and all seem good.
Read online that the stepper motor/air bypass valve could also cause this problem but my engine doesn't seem to have one of these? I think it should be located at the back of the plenum chamber but nothing there.
Any ideas would be greatly appreciated guys?????

Others on here are better qualified to assist you, but I can confirm that the 3.5efi does not have a stepper motor.

Early 3.5 on flapper does not have a stepper motor. It should have a cold start injector attached to the left hand side (when facing the engine) of the plenum at the back. if it hasn't already been disconnected - disconnect it and see what happens.

have you noticed if your fuel consumption has gone up? If it has then leaking cold start injector is likely to be the problem. you've probably already worked this out but do a search - you'll be needing to look for "cold start injector" or "ninth injector", check in here and the V8 section.

Thanks guys for clarifying that. Been looking all over for that bypass valve!
Mine does have the cold start injector. Should I just remove the electrical connection and give that a try?
I was thinking that the high revs would be down to air getting into the system, would the cold start injector allow more air or just more fuel?
Can there be a leak on the injector without anything being visible externally? I.e. would it just be leaking into the plenum as I don't see anything on the hoses.

LanzaroteSteve said:

Thanks guys for clarifying that. Been looking all over for that bypass valve!
Mine does have the cold start injector. Should I just remove the electrical connection and give that a try? Yes
I was thinking that the high revs would be down to air getting into the system, would the cold start injector allow more air or just more fuel? More fuel
Can there be a leak on the injector without anything being visible externally? I.e. would it just be leaking into the plenum as I don't see anything on the hoses. Yes

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If it doesn't improve by just taking off the electrical connector, try removing the fuel pipe as well.

Thats worth a try but I thought high revs = un metered air entering the system not fuel.
What it the purpose of the hose at the rear of the plenum? Is this an air hose? connection seems good but where does it go after the plenum?
I'm also wondering if I could have messed up the throttle potentiometer when I removed and cleaned the plenum? I didn't remove the potentiometer but the air flap inside the plenum was moved about when I cleaned it.

I wouldn't have thought that the cold-start injector would have any bearing on any idling fault, I've never bothered to connect mine as the car has always started without it, including in below-freezing conditions

Thanks Norseman, I'm thinking along the same lines but I'll give it a try later today and see what happens. It never gets below 15 degrees Celsius where I live so doubt I need the cold start injector anyway.
What normal rpm should I be aiming for? It was sitting around 1000 before but as I say now it's 1500. It also seems to race even further when coming downhill with foot off the accelerator.

LanzaroteSteve said:

I'm also wondering if I could have messed up the throttle potentiometer when I removed and cleaned the plenum? I didn't remove the potentiometer but the air flap inside the plenum was moved about when I cleaned it.

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That is a strong contender! if the throttle butterfly gap is now larger than it was, the engine will think you have your foot on the accelerator. Sorry to say it but I think you might need to just pop the plenum off again, check the setting on the throttle disc - the measurements are in the haynes manual or look it up on Rave. You can check the base setting of the throttle pot with a multimeter.

Oh and about the cold start injector - as said you shouldn't really ever need it or at least that seems to be the general consensus over time.

Looks like a good possibility. Plenum off for the third time!
I was just cleaning it! Didn't think I could have f'd something up by just cleaning something but hey ho guess I've got a lot to learn!

LanzaroteSteve said:

Thanks Norseman
What normal rpm should I be aiming for? It was sitting around 1000 before but as I say now it's 1500. It also seems to race even further when coming downhill with foot off the accelerator.

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Assuming it's an auto, 600 to 700rpm with engine warm. It will drop about 100rpm when you select Drive.

The venturi should be completely shut on idle. Idle controlled by idle air valve.

According to the workshop manual, if you measure from the mouth of the bore to the top and bottom of the throttle disc using a depth vernier, the max.difference should be 0.5mm across the diameter of the disc.

According to Haynes, there should be a 0.05mm gap between throttle disc and throttle body, this can be checked with a feeler gauge.

The throttle potentiometer should be set at 0.28 - 0.30v with throttle closed. Check scaling is smooth with no spikes in voltage and you should have a reading of something like 4.8 - 5.0v at wide open throttle.

HTH

Thanks guys I'll take a look as soon as I get some more gasket!
Anyone know what the idle should be for the manual which is what I have?

Wammers - I have the flapper model not the one with the idle air valve.