gearhead141

I own an '85 Trans Am with the LG4 which is completely stock. I rebuilt the carburetor last fall and everything was working fine. Recently the idle has been around 1000-1100 after the engine is up to temperature. When I shut the car off, the engine runs on (diesels). When the carb was rebuilt, I did not replace the MCS or TPS, and the idle was usually around 600. I have checked the TPS voltage with the key on in the last day and it is 0.64 Vdc. The problem is getting progressively worse. The other night I stopped at the gas station to fill up and the engine wouldn't go below 2k rpm in Park. Even trying to shut it down in gear couldn't keep it from dieseling. It was quite disturbing and I couldn't fill the tank. The engine starts normally and idles down after the choke warms up on a cold start-up.

Is my TPS slowly going bad? Could this be a vacuum leak? I checked the PCV valve and the plunger moves when depressed. I could really use some advice from the forum

Apeiron

Is your choke opening? Any vacuum leaks?

gearhead141

Cold start engine and choke operates normally. As engine temp starts to rise idle rises. Shut down engine and it diesels. A check of the TPS voltage shows 0.95 Vdc. I'm leaning toward a dying TPS sensor. Can anybody verify my assumption?

gearhead141

Per this post some time ago from Damon, this is what a good MCS reading is:

1. At room temperature (not hot, not cold) unplug it and check for resistance between the 2 poles of the MC solenoid. Spec is about 20 Ohms. If it's significantly higher or lower, it's shot, regardless of whether it apparently works or not.

Mine is reading 31.7 ohms which I would consider "significant". Even though I can hear the MCS clicking away when I turn the key on, I suppose it is shot?

So now I'm wondering if both the TPS and MCS are needing to be replaced? NAF, Damon, SofaKingdom, and 57Kid, I could really use your knowledge on this.

Apeiron

I wouldn't look to anything computer related being responsible for causing run-on or a fast idle.

gearhead141

The choke is functioning properly and there doesn't seem to be any vacuum leaks. The car ran fine prior to this. I'm confused that it idles normally and then the idle starts to rise as the engine warms.

gearhead141

The idle is actually around 900 when in Park. I just finished a cold start and the engine idled along at that rpm for about 5 minutes and then jumped up to 1500. This is most perplexing. When I pull off the TPS connector the idle goes up and the same for the MCS connector.

sonjaab

Check to see if the carb is loose to the intake manifold or check to
see if the base throttle plate is loose from the carb. float bowl.

gearhead141

The carb is firmly attached to the intake manifold. I did notice that the air horn screws seemed a bit loose. I tightened them all in appropriate sequence. Started the car and the same symptoms occurred. This does seem like a vacuum problem. It happens after the same amount of running time. The choke doesn't even open fully before this happens.

gearhead141

I'm thinking I've got a TPS going bad. It is original to the car. The car sat for ten years. I rebuilt the carb last October and it's been working fine up until a couple of weeks ago. I'll let you all know how it's running afterward.

five7kid

iTrader: (14)

It all sounds to me like the high idle cam is hanging up.

naf

iTrader: (7)

Bad TPS won't cause a high idle unless it's engaging the idle speed solenoid, which would otherwise be apparent.

When you pull the plug on the TPS or the MCS the carb responds by going full rich. An increase in idle speed concurrent with this action could be a vacuum leak.

Please don't start swapping sensors before you've confirmed the simple stuff by diagnosis.

gearhead141

I capped off all the vacuum ports on the carb, and started the engine cold. Engine speed goes up to 1500rpm. I plugged the intake vacuum port behind the carb and started the engine again; no change.
I can move the choke flap towards closed with my hand and the idle goes down. Would this indicate that the choke is out of adjustment?

gearhead141

I guess I'm going to pull the carb off and check that fast idle cam and linkage. It doesn't even seem like the throttle valve is making contact with the cam. From the little I can see behind the choke t-stat, the cam is next to the throttle piece, when I think it should be in line to make contact with the steps. Don't know how that could have happened, but would that be the cause of the high idle?

gearhead141

Got the carb off and found that the fast idle screw was too far out. The throttle wasn't even making contact. Since I've got it off I'll check the other adjustments. Any advice from the forum as to checks to perform?

cumminsN14

did you figure it out? mine is doing the exact same thing.

EnvoyIROCZ

mine does the same thing I just give it a quick rev and the idle goes down

gearhead141

I haven't figured it out yet. I bought a pack of vacuum plugs and blocked off all the vacuum ports to see if there was a leak. The car did the same thing. So it is either the choke or there is an internal vacuum leak, i.e. idle air bleed. I'm going to get it to a local carburetor shop and have them check it out. I will post results when it is fixed.

cumminsN14

the cam is shot in mine, everytime id flip it back down and hit the throttle it would diesel back up, basically it was trying to run away on me like a diesel with to much fuel. short term solution for it.... zip tied it till i can rebuild the carb. idles great, runs smooth. this is on a 82 which for it never being rebuilt im impressed how smooth it runs since my 86 was a rough sob. by the way it would rev to 4k plus.

gearhead141

I'm starting to think I may have the same problem; fast idle cam is worn out. Over the weekend I removed the choke t-stat and started the car. Idle holds at 1500-1600rpm and no amount of blipping the throttle has any effect. The fast idle cam, at least the part I can touch is pushed all the way down. What else can I do to correct this problem? When I put the trans into gear, the rpm drops down to approx. 1100rpm which is still too high. I figured while I had the choke t-stat off I would measure resistance across it. I got 10 ohms, so I guess that's working fine.

gearhead141

Here is an update on my problem: the E4ME was professionally rebuilt. I installed it and the problem remained. The engine is still at fast idle of 2200 rpm. Stabbing the throttle has no effect. So this morning I disconnected the four-wire plug (EST connection) and started the car. No more fast idle, in fact I had to give it some throttle to keep it running. This was a stone cold start. My question to the group, is this an indication that the ECM or distributor are bad?

naf

iTrader: (7)

not at all. when you disconnected the EST you removed the ability for the ECM to advance the timing. timing went back to base setting and the engine ran lazy. perfectly normal reaction to the EST being disconnected. this disconnect will even trigger the idle bump solenoid to engage to boost idle to compensate for the loss in timing (providing the solenoid is functioning).

gearhead141

But that was my point, perhaps the ECM is advancing the timing too far. Which is causing the fast idle condition. Does that make sense?

naf

iTrader: (7)

no. the ecm adds around 20 degrees of advance to base timing after the engine starts. it's not going to suddenly start adding more and if you have the base timing so far that timing is an issue, the car won't be driveable.

look to your throttle blades. if you can lower the idle by manually closing the throttle (at the carb) your throttle bores may be worn allowing the butterfly to hand slightly open.

gearhead141

Just wanted to update on the carb situation. I took the car back to the shop that did the overhaul. They're a very stand-up shop and guarantee their work. I was called near the end of the day and told it was ready. When I picked the car up this morning it was that familiar normal idle sound. I asked the technician what he found. He stated that the secondaries were slightly open or hanging up. There was no charge for this as I had brought just the carburetor in for the rebuild. For any of you who are in the North San Diego area, these guys are very good with carburetors. The shop is Certified Service Center. I have no affiliation with them but feel they should get a good word.

By the way NAF, you were right about throttle blades.