Cold idle problems

[rcturgeon]

I got an 88' 2.5l TBI (sportruck if it matters) about a year and a half ago its now got 213,000KM on it and i worked on it for a while to get it road worthy, just about everything is running great and I only have one real issue left and its getting it to start and idle when its cold.

I've read posts from others for a long way back, but alas, i still do not have a solution.

 

so the breakdown is:

- start truck and after about 2 seconds it dies

- i can play with the gas pedal for a minute or so (longer if its really cold out) and it will run fine after it warms up

- once its warmed up for a bit it seems to run great as far as i can tell

- ive got new plugs, plug wires are great, the distributor looks good, the coil is good, gas flow and pressure seem fine because its fine when its warm, Ive gone over vaccum lines countless times to find leaks and even used propane to blow over all the lines while its idleing to find leaks, MAP sensor is tested good, I looked at the EGR valve and checked the mobility of the diaphram and if i hold the diaphram in while its warm it stumbles and runs rough I'm sure i've done more but i can't remember at the moment

-the only thing i havent done is checked the compression but I'm not sure if that would be affecting it in this way even if it was low, but i suspect its good

- ive read tons of posts and service manual tips and just about everythign else but i stil can't find something that works.

 

If there are any other symptoms that will help diagnose please ask, I'm getting desperate now

[jeepcoma]

I'm not familiar with the specifics of the 2.5 but the following generally applies to EFI engines..

 

If you were running a carb I'd say to check the choke, but since it's EFI I'd check the coolant temperature sensor and also make sure that the high idle speed is correct (using an external dwell/tach, not the factory tach if yours has one). I'm not sure what the voltage specs are for the CTS but they must be out there, check it when it's sat overnight and it should be pretty close to the spec for the outside air temp. The Intake Air Temperature sensor could also be faulty but I think it'd take more than a minute or so for it to not stall when you're not playing with the throttle.

 

How did you test the MAP and know it's working? Something to test is to unplug it and force the computer to run in speed density, which would eliminate the MAP sensor as a variable.

 

Just for kicks, remove and plug the vacuum line from the EGR for testing. It removes one more variable. Make sure it's 100% closed, it doesn't take much for it to affect how the engine runs when cold.

 

How do you know fuel pressure is good? You need a gauge to diagnose, nothing else will do (even if it "seems fine").

[1987Comanche]

I'd say 90% chance your Idle speed controller is messed up or inoperative. My '87 2.5 had identical symptoms.

[rcturgeon]

good thought 87' but the idle speed is all done automatically and it can't be adjusted, and once its warm the idle is where its supposed to be, I'm not an expert by any means and i know for sure everyone else here is probably more knowledgable than i dream to be. ill look at the suggestions from you coma

 

as for the MAP i checked the ground voltage and it was between the recomonnded 4.5-5 and when it was idling and warm i checked it again while it was idling and it was lower somewhere near 2 or something and it flucuated with the RPM changes .. and when i unplug it to run it in 'high speed density' but it would stumble even quicker and when i played with the throttle it was a hard job to keep it going at all

 

okay i just looked up some numbers and a test for the CTS so ill do that inbetween classes tomorrow and find out if thats it

thanks for getting back to me guys

[1987Comanche]

The idle speed is computer controlled and automatically adjusts if the ISC is working properly. It's a stepper motor on the Throttle body that physically pushes the throttle open when cold and you can adjust the length of the screw if you loosen the locknut. I'm guessing it's seized and was adjusted so the idle is right when hot.

Try this. Unplug the ISC (a 4 pin connector if I remember correctly) and see if the truck runs differently. I bet you won't see any change at all.

[rcturgeon]

well 87' i know what your talking about and ya the screw and lock nut looks as one because its all rusted up.. but the actually moves when the truck is started and shut off. pushes out on the throttle when its shut off and just after it starts it will pull back in .. but when i unplugged it before it would idle way higher i believe.. actually started easier when we unplugged it but like i said before it would just idle really high after it started.. I'm not sure if its supposed to be a progressive retraction when it starts or soon as it starts just pull all the way back almost immidiatly.. like i said.. I'm new at alot of this.. but i learn fast, read alot, and in no way discredit anyone with any information

[rcturgeon]

okay so i did some tests with a digital ohmeter on both the CTS and the MAT sensor and buth of them came back infiniity even with the engine running at normal temperature, ill try replacing these but it will take a while to get the parts in up here so ill tell you all how it goes

 

thanks for your help guys

[Jakeman17]

my 4.0 does that also

[WBKrazy]

mine did the exact same thing, i'd have to keep the gas pedal down a bit for a couple minutes until it would run under its own power.... did it during the winter, the summer and all points between, didnt matter what the temp was.... chased it for months, replaced every sensor, the egr valve, and the idle stepper, eventually i just gave up and adjusted the idle a bit and dealt with it

[rcturgeon]

well guys turns out it was the CTS thanks for all your help