This is an ancient thread but I didn't want to start a new thread based on the same issue. I have a 91 GMC Jimmy which idles horribly (and it has even stalled) when it is cold outside (17* C or less) and it is the first start of the day. It idles rough for about 20-30 seconds (longer when colder) then smooths out to a normal nice smooth idle. I have been data logging and have a reference log if anyone is interested. Everything on the truck is in a stock configuration. At this point I have done a few things and have a few more in mind.
Things I have done,
1. Data logged, on the rough idle start I have noticed a couple things, the IAC is almost closed during the bad idle and when it begins to open the idle smooth out. I assume this acts like a choke and needs to be closed like this. Also when the truck is about to die the vacuum drops way down (14-15). I don't know if that means there is a leak or simply the truck is just not pulling enough vacuum at the time because its about to stall out.
2. Coolant Temp Sensor reads good on all data logs
3. Smoked the vacuum system for leaks, non found other than the seal above the TBI Throttle body and right below the filter housing do leak, I didn't think this should affect the vaccum. Also an EGR block was installed for troubleshooting and really didn't change anything.
4. IAC was pulled, looked serviceable and ohms check was good right at 50.
5. I hear the fuel pump with the key on and the spray patter from the injectors match each other.
Things I was thinking of doing,
1. Fuel Pressure gauge from the parts store to test for good PSI and no blocked return line, most of my data logging the BLM data is returning slightly rich numbers so I may be too rich open loop on the first start of the day.
2. Pull spark plugs, I plan on getting to this but the entire ignition system is only 2,000 miles old and looks to be of reputable parts (this was installed before I bought the truck)
I am open to checking any other suggestions, I haven't replaced any parts yet, I kind of wanted to confirm a bad part before I replace something. I will post more was time goes by. Thanks in advance!
Not cracked but had a small hole in it and the little rubber diaphragm inside had seen better days. The part cost me $5.65 - up just a little, but we'll blame that on inflation - but she idles and runs like she should now!! Seems to accellerate better too. I've been looking for that leak for a long time now!! Amazing how big of a headache a little itty bitty P.O.S. $5.00 or so plastic part can cause! That takes care of one, now on to the next.
