low idle and vacuum issues

[DSMxTSi]

We've been having a low and rough idle on our 97 GST for a while now. Any acceleration/cruise over idle is fine. Vacuum only pulls about 10 inches at idle. We tried a boost leak test but could only find that the connection at the tester itself was leaking. So now I have rtv sealant setting up on the tester as I type this, and will try that again later.
So I just checked the ISC and it appears to be bad. It only reads.03 ohms, but the car had been running earlier so it isn't entirely cooled off yet. Though I doubt that it should read that low being a tad warm. Will a bad idle due to a faulty ISC cause the vacuum to be off? Is the ECU thinking it's not at idle so it tries to compensate? It does run pig rich at idle and it sounds like there are small rocks in the fuel rail. It's hard to describe the sound but small rocks is the best I can come up with, or maybe an electrical sparking. So I checked the resistance on the coil pack, the primary is bad and the secondary is good. If it was really bad there should driveability issues.
The CEL is for the Speed Sensor and I changed that with no effect. Also have a MAP sensor CEL.
So now I'm scratching my head. Why is the ECU throwing different codes when the ISC and the coil pack check out bad. Could it be a bad ECU? I pulled the ECU a few weeks ago and the CAPS appeared to be ok, plus there's no fishy smell.
This has been our least modded car and is now giving me the most problems. Go figure. :rolleyes: It only has a drop-in K/N, Brode upper i/c pipe, and 1G bov.
If it idles for an extreme amount of time it will start smoking out the exhaust really bad. This should be indicative of the valve guide seals going bad. But would bad valve seals cause a bad idle?

[DSMxTSi]

Ok, just did a compression test. While it was warming up I pulled the ISC connector and there was no change in idle. I'll run to Mitsu and get a new ISC tomorrow.
The compresion test readings have me stumped. They are:
#4 = 200 psi
#3 = 170 psi
#2 = 180 psi
#1 = 205 psi

2 and 3 seem fine, but WTF is up with 1 and 4. Should be about 180 tops, so 1 and 4 are too high. The plugs are fairly new and all are fouled pretty good. There's no oil on them.

[Jason]

My guess on the high compression on 1 and 4 is oil buildup on the tops of the pistons from leaking valve seals.

Just a guess of course.

[DSMxTSi]

My guess on the high compression on 1 and 4 is oil buildup on the tops of the pistons from leaking valve seals.

Just a guess of course.


Thanks Jason. Yeah, I was just searching DSMTuners and came across a post that mentioned excessive carbon buildup can cause high compression numbers.
Would the bad seals cause the rough idle? I'm possibly looking at more than one problem, hopefully the new ISC tomorrow will fix the idle. Then I'll decide if I can afford a rebuild on the head.

[dvs_dsm]

Did you try running any fuel treatment through it to try and clean up your fuel problem? Thats something thats cheap and shouldn't hurt it in anyway.

[DSMxTSi]

Did you try running any fuel treatment through it to try and clean up your fuel problem? Thats something thats cheap and shouldn't hurt it in anyway.

I think I ran some Lucas fuel system cleaner in it a while back.
I've known that the valve guide seals have been bad for some time now after I had to jump start the car and let it idle for approx 30 min to charge up while I showered. It'll sometimes smoke at start up too, though not that often.

[Dane Becher]

Would the bad seals cause the rough idle? I'm possibly looking at more than one problem, hopefully the new ISC tomorrow will fix the idle.

Highly unlikely valve seals are causing idle problems. Do you have A/C installed? If so, what happens to the idle when you turn it on? With a good ISC the idle should increase slightly when A/C is turned on.

Now if it's an issue where you simply cannot set the idle low enough (constantly idles 2k+ and may surge), it could be your FIAV (which is likely on a high mileage DSM and 1gs). Unfortunately there is no testing proceedure for the FIAV besides blocking or swapping it.

Maybe this helps...

[DSMxTSi]

Highly unlikely valve seals are causing idle problems. Do you have A/C installed? If so, what happens to the idle when you turn it on? With a good ISC the idle should increase slightly when A/C is turned on.

Now if it's an issue where you simply cannot set the idle low enough (constantly idles 2k+ and may surge), it could be your FIAV (which is likely on a high mileage DSM and 1gs). Unfortunately there is no testing proceedure for the FIAV besides blocking or swapping it.


A/C is still on the car and couldn't honestly say whether it affected the idle. It's been a while since I last drove it in warm enough weather. It must have seemed normal or I would've remembered something.

The idle has never surged like the typical DSM with a bad ISC. That's one reason why I just got around to testing it yesterday. I just ordered a new one from Little Rock Mitsu but it won't be here till Wed or Thur. Whew, $213 for that sucker!

[Jason]

Highly unlikely valve seals are causing idle problems. Do you have A/C installed? If so, what happens to the idle when you turn it on? With a good ISC the idle should increase slightly when A/C is turned on.

Now if it's an issue where you simply cannot set the idle low enough (constantly idles 2k+ and may surge), it could be your FIAV (which is likely on a high mileage DSM and 1gs). Unfortunately there is no testing proceedure for the FIAV besides blocking or swapping it.

Maybe this helps...

I agree. Valve seals typically don't cause idle problems.

I think you're on the right path with the ISC.

[DSMxTSi]

I agree. Valve seals typically don't cause idle problems.

I think you're on the right path with the ISC.

Yeah, I hope so.

I had a knowledgeable person (or so I thought) that told me the bad valve seals could cause a bad idle. So I was kinda looking at one problem when I apparently have at least two. I hope the coil pack is ok, but I would like a coil-on-plug.

[DSMxTSi]

I found a new website that sells brand new genuine Mitsu ISCs for $129.75.
Needless to say, I cancelled my order at Mitsu and am ordering one from dsmisc.com.