95' 3.4 DOHC High Idle

[dlaroche]

Hey everyone,

I got a 95' Monte Carlo with the 3.4 DOHC, and it runs fine except this weird idle problem that it has. When you start the car it might idle "ok", but most of the time the idle jumps to around 1500 RPM, and sits there for a few minutes, and then returns to about 800 RPM's for a few minutes. It will then go back to a high idle for a few minutes, and then again return to a normal idle.

The car will do this regardless of whether you are just sitting in park or driving it. Of course when you are driving it the RPM jumps are a little less, but are noticeable. I do have a check engine light, however, since it is a 95' with the transitional OBD system in it I can't readily read the codes.

Anyone have any suggestions to what the cause or remedy may be?

Thanks in advance,
Dustin

[tinindian95]

Go to Autozone if you have one nearby and have them throw the scanner on it and get the DTC code or codes. Then go online and do a google search for the DTC and go from there.

[dlaroche]

Hey,

Thanks for the suggestion, but taking it to Autozone is out of the question for a couple of reasons. One, the nearest one is like 2 hrs away. Second, I don't trust the car to be driven on the highway like this, especially on the interstate which is our main road of transportation out of this very small town I live in.

I have ordered some electronic parts that should be here tomorrow that I will assemble so that I can hook my laptop into the OBD port to read the codes.

Also, from my understanding most of the auto stores aren't equipped to read the OBD "1.5" computers anyways.

[95SleeperAcheiva]

My '94 did the same thing and didn't pop a single code. I found a slow IAC. Still check for the usual, like vacuum leak etc before changing it.

[bbeck]

intake manifold gaskets go bad- just like the rest of the motors of the era. Only thing is, these don't have coolant pushing through them to show they're leaking. Be sure to get at the o-ring at the distributor shaft while you're in there

[Dave96z34]

95s are a bitch to scan they are obd1.5

[dlaroche]

Dang, thanks everyone for their responses. I failed to setup my account correctly to default to subscribe to threads so I didn't realize I had any responses. I apologize for my ignorance.

However, I did end up making a two-transistor cable that read the codes fine using TunerProRT. The only code that I get is P1650 which is related to the Quad Driver #2 having issues. I looked on Alldata, and found out that this circuit controls the EGR, and some solenoids on the transmission. I also noticed in TunerPro that I didn't receive any feedback for the EGR solenoids or the tranny for that matter. I plan on testing this out further.

I also noticed that the car is saying it is running lean, and it does not fluctuate back and forth from lean to rich as Alldata states that it should be doing. So this indicates a BIG vacuum leak to me. What is your feedback on this reading? I plan on going out tomorrow to spray some WD-40 around the intake, EGR, and vacuum lines to see if I can spot any leaks. I will keep you posted on that.

I have already taken the IAC valve off, and cleaned it with carb cleaner, but did not notice any changes. I figured if if was slow or stuck that this should have freed it.

[Schurkey]

WD-40 wouldn't be my first choice for an aerosol vacuum leak detector.

Aerosol carb spray is what I use.

[dlaroche]

I'm new to TunerPro so I miss interpreted the information pertaining to the whether the PCM was constantly switching from Lean to Rich as Alldata stated. After running the scanner again I noticed that it was doing so properly.

However, I did notice that the computer was saying the AC Clutch was engaged when the cabin controls were off. So I disconnected the plug at the AC Compressor to see if this made any difference, but it didn't. The PCM is thinking the AC Clutch is engaged. Even when the plug was disconnected. So is there another sensor involved that may tell the PCM this information.

I also noticed that the PCM was commanding the car to run at the ~1500 RPM range, and the engine is doing so. My theory is that since the computer thinks the AC Clutch is engaged then it is commanding the car to a higher RPM. This could be incorrect, but that's kinda what I'm thinking on this now. I will dig on Alldata some more to see if I can find another sensor that would be telling the PCM this information.

If I post a data log of from TunerPro could someone play it back and take a look at it for me?? I'm sure that someone who has experience with interpreting scanner data could spot the problem pretty quickly.

[bbeck]

I'd still put my money on the intake gaskets if you haven't already done them on this motor. What kind of mileage are you getting? Mine read it was running lean as well while I was pulling at best 18mpg until I did the intake gaskets and started getting about 25mpg and the codes stopped popping up. My ex-gf's car did the exact same thing.

[dlaroche]

Yeah you are correct I think. I decided to pull the intake off, and assess the gaskets. They lower intake gasket was shot! It was missing the rubber seal on two different cylinders. I had to order in the gasket set, and hopefully pick them up today. I am almost sure that this is the cause of the high idle now.

While I have the intake off is there anything I should do to the engine now?? Like the oil pump drive seal or anything? I think my oil pump drive seal may be developing a slow leak. Not sure though as I haven't really driven the car since buying it.

The car only has 124,000 miles on it, but I think the three previous owners were teenagers so I am sure they were pretty hard on it. I know they were at least to the exterior.

But the engine looks and sounds good besides the high idle. No knocks, ticks, or anything of that nature so I am sure it still has plenty of life left in it. Plus I only paid $350 for the car so not a bad deal in my book.

[bbeck]

definitely do the distributor hole cap- mine started leaking with about that many miles pretty bad- right after I did the intake gaskets. It's embarassing the amount of oil that thing was leaving behind in driveways. might want to look into the doing the timing belt at the same time as well if it hasn't been done recently

[dlaroche]

Well, I finally got the lower and upper intake manifold gasket replaced. It did help the idle significantly. However, I ran into other issues now.

Now I get an intermittent check engine light, and the codes that are getting thrown are P1640 P1650, which both are related to the quad drivers in the PCM. The P1640 is the one causing the intermittent check engine light to come on. Sometimes the code is thrown, and sometimes it's not.

I went through Alldata, but couldn't find any information to what the quad A driver controls so I can troubleshoot them.Does anyone know what P1640 is related too??

[95SleeperAcheiva]

I have identifix, I'll check when I get to my shop and see what pops up. ETA 2 hours.

[95SleeperAcheiva]

Check the Brown wire at pin E of the transmission connector. If no voltage is present, then check the TCC fuse in the right underhood electrical center. If there is voltage at pin E of the transmission connector, check the wiring from the transmission to the Powertrain Control Module (PCM). Check for any signs that wires might be shorted around the AC accumulator.

If these codes weren't there before your repair it's something you did.

Trust me, I've been there. Had a bad yet running motor...rebuild it, then spent 3 days looking for why I had no fuel delivery, all because of a ground wire that was hiding behind the motor and didn't reattach.