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

[GOT2B GM]

Well, I would have the code read so you aren't guessing and throwing parts at it.

Could be a bad IAC valve, TPS, an intake manifold gasket leak, a big ass vacuum leak, hard to say.

Take the gift the car is giving you (the code) and use it to your advantage.

[dlaroche]

I don't think just throwing parts at it would be the best idea either. However, I live in a very rural area, and the closest town is around an hour away that could possibly read the codes for me. Due to the instability of the idle, I don't trust the car to drive it that far let alone on the interstate like that.

That is why I am asking for help here. Maybe someone has ran into a similar problem, and found the remedy to it. If so, then I wouldn't just be "throwing parts" at it, but going with educated guesses. Also, if I knew what sensor was causing this then I could possibly check the sensor on a similar vehicle before buying a new part. Again, eliminating just "throwing parts" at it.

[Andrew]

Like matt said, it could be several things, so getting the code is the easiest way to narrow it down.

[dlaroche]

Taking the car in to get the codes read is not an option for me. However, I just checked the tracking of my electronic components that I ordered, and they should be here tomorrow or the next day at the latest. I will then just assemble my own OBD scanner using the MAX323 chip, and my computer.

When I figure it out I will post the results so that maybe somebody else can find an answer to similar problems.

[White93z34]

it could be lots of things from a bad IAC to a vacuum leak, to a cracked lower intake gasket.

[oz37k]

I had the same issue on my 95 Z, check the rubber connections that join the hard vaccuum lines. Most of them are either rotted or leaking from small cracks. That fixed my issue. Also any rubber emissions hoses that you see, get those too. Then attack the intakes cause most of the time it is a cracked gasket like the above.

[Galaxie500XL]

Had the same problem with my "95 LQ1, twice. Turned out to be a bad throttle position sensor. The ECM thought the throttle was open to around 6% on startup. The replacement sensor died in less than two months, so I replaced it again..no further issues in the past year or so. But, you really need a scantool that will show live data to figure that out..

[Psych0matt]

You say you don't want to just throw parts at it, and we're trying to help, but there are so many different things it could be. Get the code read so we know where to start. OR throw parts at it.

I had a similar problem, but it was my intake gaskets.

[tornado_735]

I'm willing to bet it's the idle air control. Mine just died with similar symptoms. The sensors are pretty universal between GM engines, should be cheap enough to find one.

[jeremy]

I had a similar problem, but it was my intake gaskets.

X2

[ManicMechanic]

X3 on the intake gaskets.

[GnatGoSplat]

X3 on the intake gaskets.

X4. Exact same symptoms as you describe. I verified mine by spraying carb cleaner at the gasket and could hear change in RPM.

[dlaroche]

Alright everyone,

I bucked down, and replaced both the lower and upper intake manifold gaskets. That improved the idle significantly! The old lower intake gasket was missing the rubber seal in spots on two different cylinder intakes. Thanks!

However!!! I built a ALDL cable that allows me to run TunerPro to read and diagnose the engine codes. I am receiving a constant P1650 code, and an intermittent P1640 code. They are both related to quad driver issues.

The main one that I would like to diagnose and fix first is the P1650 quad b driver code. According to Alldata, this circuit controls the following:
Shift Solenoid A and B
EGR

Alldata instructed me to turn key off, disconnect all plugs at pcm, turn key on, and then test certain pins for the amps being put out. All my readings should be more then 0.4 amps, but less then 1 amp. However, on pin 21 of the A connector I get a reading of 2.05 amps. This is the shift B solenoid according to the diagrams. My indication is that the solenoid is shorted out.

So is there a better way to test out the solenoid without tearing into the transmission first?? I understand the solenoid is only a $10 part, but then you need the gasket kit, and all the labor involved in getting at the tranny. I would just like to have piece of mind when I tear into the tranny that I know I'm doing the right thing to resolve the problem.