[sprocket]

Hi everyone. I have an '88 Eldorado that I have done almost everything to to improve the curb idle. Here is a list of what I can remember that I have done:

Adjusted timing following the proper technique.

Changed ISC motor and adjusted.

Installed stiffer throttle spring. The ISC motor did not function properly with the original spring.

Changed cap, rotor, plugs and plug wires. Installed high end plug wires that got great reviews from several sites. Can't remember the brand name now. Gap on plugs is correct.

Fixed all vacuum leaks and checked with vacuum gauge.

Changed PCV and grommet.

New intake gasket set.

New gasket under throttle body. New throttle body gasket set. New fuel pressure regulator.

New EGR gasket. EGR seems to function fine.

Map sensor is in proper parameters.

Adjusted TPS.

Changed fuel filter. Ran Redline fuel system cleaner through system and then changed the fuel filter again.

O2 sensor is in proper parameters but it MIGHT be sluggish.

The CAT is not the problem. I won't give details but trust me.

There are no codes. System goes into closed loop. Diagnostics must think the engine is running fine.

Car idles around 850 but bounces around about 50 RPM.


What I am still looking at:

1) Maybe it's the injectors. I have a spare set I could send to Witch Hunter for cleaning.

2) I read somewhere that it could be a grounding issue. The engine has been replaced. There is a body to engine ground strap that runs over to a nut on a strut mounting bolt. Maybe that is a bad spot. I think there should be one down by the starter but I'm not sure where. Is that correct?

3) Fuel pump? Fuel pressure could be low. Hard to tell without a throttle body fuel pressure gauge. Standard gauge that reads to 100lbs is too hard to tell.

Sorry for the long post but I wanted to cover what I have already done.

And ideas would be appreciated. Thanks for reading!

 

[Ranger]

The injectors or fuel pump would not cause a high idle. If you suspect a ground, run a redundant ground using a jumper cable from the negative battery post to a known good ground.

Have you pulled the DTC's?

 

[sprocket]

Is 850 RPM high? It usually drops into the 700's when in gear. The idle is not smooth. The RPM bounces around. A poor spray pattern from the injectors could surely cause a rough idle, right?

There are no codes except related to the AC. m The AC pump is disconnected because the AC lines were removed when the replacement engine was installed and haven't been put back on yet because I don't know the proper procedure to do so and get the AC working again.

I will try grounding off the battery like you suggested.

Thanks for the reply, Ranger!

 

[Ranger]

What engine? 4.5? I'm not sure what the spec'd idle RPM is, but 850 sounds a little high. An injector could cause a rough idle, but not a high idle.

 

[sprocket]

It normally would have a 4.5L but when it was replaced, a 4.9L was installed. Externally they appear to be identical. I had to use the intake and distributor from the 4.5L because the 4.9L is not throttle body injection. All of the electronics and wiring are based on the 4.5L with throttle body injection.

Maybe the 4.9L conversion has something to do with the rough idle.

 

[Ranger]

I'm stumped. Sounds like you've changed everything there is to change.

 

[sprocket]

I'm bringing back this old thread.

I've done even more to improve the idle on this car but the idle is still not smooth. I've installed an OEM ISC which got the idle at the right RPM range with no surging but the idle still bounces around about 50 RPM.

Oh, and just to clarify, this car has had a 4.5L to 4.9L swap while using the original intake. So it is a TBI 4.9. The computer thinks it is a 4.5. I am currently working on finding someone who can program me a new chip or at least tell me if it is even necessary.

I have experimented with 5.0L injectors. The car is much more responsive but this does not cure the idle roughness. It helps a bit. I stopped using them because the computer still thinks they are 4.5 injectors and the engine runs dead rich until the O2 sensor warms up. Then it corrects and goes into closed loop. Since it doesn't cure the idle problem I put the original 4.5 injectors back in. There is no doubt they need cleaning or replaced. Maybe that would solve the idle issue but I really doubt it.

So here's what I am wondering: Is the longer stroke of the 4.9 messing up the idle? The computer thinks the engine is a 4.5 which has a shorter stroke. I do have trouble setting the idle spot on because it is never steady on the timing mark. Is that because detonation never occurs exactly when it should because the computer is set up for a 4.5?

Any comments are appreciated!

 

[Ranger]

WOW! A TB injected 4.9 with a 4.5 PCM. I wouldn't know where to start anymore. Maybe put the 4.9 manifold with multiport injectors back on and use a 4.9 PCM? :noidea:

 

[brougham]

It's probably something related to the different engine size. That's the problem with doing engine changes on computer controlled cars.

 

[sprocket]

WOW! A TB injected 4.9 with a 4.5 PCM. I wouldn't know where to start anymore. Maybe put the 4.9 manifold with multiport injectors back on and use a 4.9 PCM? :noidea:

I'd be all for doing that but it looks like it would be a royal pain to change the wiring harness. It has an awful lot of "tentacles".

 

[Ranger]

I hear ya. That might be another insurmountable problem. Brougham hit the nail on the head.