Idle and Hesitation issues

[Black92GST]

Hello, I am a pround new owner of a 92 gst.

First of all, my rpms jump up and down at idle, so i believe there is a vacuum leak. It seems like all the rubber hoses need to be replaced, so I will probably do that soon.

But the second thing is, when free reving the engine, if I do partial throttle from idle, the engine will die, then rev up. WOT seems to be ok.

usually people on the forum have problems with WOT or higher rpms. I have a problem with parital throttle, and leaving from a stop. the hesitation at part throttle makes the car stall when letting out the clutch.

so is this a TB, IAC, MAF problem? or can a vacuum leak cause a part throttle issue?

Here are the things I have done to the car since bought:
- Engine Oil and Filter Change
- Trans Oil Change
- Changed the spark plugs to NGK BPR6ES. the old ones were NGK Iridium BPR6EIX, they were black but the tips were light brown)
- Previous owner Changed the Timing belt, thermostat and water pump

Thanks for the help

 

[DSM4me44]

Make sure your spark plug wires look good and doing a boost leak test would be the next best place to start.

 

[Gstclipse69]

As said before, boost leak test and wires. I would also check out the throttle body to see if the plate is sticking, and check to see if the throttle cable has any slack.

 

[Black92GST]

ok thanks guys.

Today, I first checked the spark plug wires. They are pretty old looking 7mm wires with no visible company name. The insulation did not have any rips or holes though.

The ohms were:
1. 5.83 kOhms
2. 5.58 kOhms
3. 6.76 kOhms
4. 8.08 kOhms

(oh by the way, i guessed the 1 cylinder was one closest to the front or crank pulley side.)

seems a little high, and variated.

Then I boost leak tested the engine. I used the vfaq build to make my own tester.

Well I put the leak tester at the entrance to the compressor. It didnt hold any pressure, with a leak sounding from the compressor itself. There is also an oil leak that is apparent in the area between the compressor and turbine.

im guessing the oil leak is from that bolt on top of the turbo, the oil line. Maybe the boost leak is from the intercooler pipe coming from the compressor? is there a gasket between the pipe and compressor?

but the funny thing is, when I test drove it and drove it home, It made 15lbs of boost (with MBC) with no leak.

The throttle cable is tight and returns just fine. I haven't yet checked the throttle plate itself.

So my boost gauge fluctuates between -16psi to -19psi at idle. I'm thinking I have a vacuum leak for sure. Maybe multiple problems. I don't know If a stuck open throttle plate or bad IAC could cause idle to fluctuate frequently like it is.

Well I found a vacuum hose kit on the RRE website for $40. Is this a good product? good price? any other options other than Kragen?

Thanks again.

 

[decathect]

To help you out,

There is a metal gasket between the compressor housing and charge pipe.

If you can't build any pressure in a boost leak test, that isn't good. My car held pressure for a good 10-15 seconds before fizzling out. It is possible you can still hit 15 psi, but you're overworking your turbo like crazy and you WILL have idle issues due to it.

based on your problem with wanting to die at part throttle, it sounds like its running rich. What kind of gas mileage are you getting?

 

[decathect]

also, check out this page, I found it helpful:

http://www.geocities.com/motorcity/downs/7678/dsmtests.html

 

[Dsmj89]

Have you tried wetting your intercooler pipes with soapy water and see if any bubbles form with the car on ?
Also if you lightly press the gas and your car dies, it could be also a dirty fuel filter and/or fuel lines. Get this checked out and report back.
Most people oversee this small little piece that is 10+ years old.

 

[Black92GST]

Ok, I had time to test some stuff today.

First, I tried to do the soapy water test on the intercooler hoses. Honestly, I didn't see any bubbles. I put soap water in a bottle, poured it at many places, and turned the car on. It doesn't help that the previous owner wired the radiator fan to be on at all times either.

But some good news:

I tried the electric tests on the ISC Motor, Idle position switch, and TPS that decathect found

http://www.geocities.com/motorcity/d.../dsmtests.html

The ISC motor first:

1-2 read 33.6 ohms
2-3 read 33.3 ohms
4-5 read 33.6 ohms
5-6 read 1.4 mega ohms - so this is bad. basically OFL.

so....I'm going to try to find a used ISC motor. =)

The Idle position switch was ok. It read continuity to ground when closed, and OFL when the throttle was open.

The TPS was also ok. 1-4 read 4.6 kilo ohms. 1-3 changed readings smoothly as the throttle was opened.

to sum it up so far, I need a new ISC motor. BUT I still have to find out why I can't hold boost in the boost pressure leak test. Maybe I'll block the fan with something and try again. Oh yeah, and change the fuel filter as well for maintenance, but I want to try the ISC first.

 

[decathect]

ha! thats the same coil that was bad on my ISC. I would put highest priority on getting the boost leaks solved, because that is guaranteed to solve you problems.

 

[decathect]

oh, by the way, I might have a spare good ISC that I could sell you. Let me know if you're interested.

 

[mieszkoj]

Boost leak more likely, went through the same problem last year.. Leaked form the BOV.. also this could happen if your are venting the bov improperly.

 

[Black92GST]

thanks for the offer decathect, but I'm going to search for it locally first.

by the way, on my car, the charge pipe right after the compressor housing had a gasket that seem like paper. I just peeled it off by hand as it broke into pieces. I remember you (decathect) mentioned it was supposed to be a metal gasket. So...was the wrong gasket on there?

 

[1SickAuto]

i would check the BISS screw. mine did the same thing, would hesitate with the slightest throttle and sometimes die coming to a complete stop.

 

[Black92GST]

the biss screw is the idle screw on the TB right? It is still there, and it works. I turn it one way, the idle gets higher, and I turn it the other, idle lowers. I noticed that if I turn the idle up so it's over 1000 rpms, I don't have the hesitation at part throttle as much.

I just found this site:

Terry's Talon Troubleshooting Tips #3 - Fast Idle (aka. Idle Surge)

 

[1SickAuto]

yeah the biss screw has to match the idle or u will have that problem. i just got a tuned chip and i had the the guy set a custom idle at 1k and b4 it was set to 750 and it would do that until i set the BISS right. oh by the way i would see if u cas is set right too. if u have a logger it should be 5 btdc

 

[Black92GST]

I drove the car on the road today to see how much it was boosting with the stock boost solenoid back on. It held at 7psi.

I also found out the gasket after the compressor is acutally made of rubber. Mach V sells it for like 4-5 dollars, and thats what they told me.