car idles cold but not hot?

[vetteinaz]

I recently purchased a 84 crossfire and i am trying to tune it up. I've read over information here but haven't found this problem yet. The car ran perfect which I'm told from the people i bought it from/still helping me work on it 14 years ago. It sat for 14 years and had 132k on it then tho. It was running really rough when i got it but changed out the o2 sensor and its running great except with a hesitation when I punch it from idle range. I replaced the fuel pump when I got it but with the stock one before I started reading about the 85 swap(hopefully this will fix the hesitation). just this week though it has been having a hard time starting when warm or hot but not cold(it didn't when i got it). and it idles perfectly at 800 rpm park and 700rpm when in gear when cold. but as soon as it warms up and I touch the throttle it will most of the time flutter and stall out. I can sometimes get it to idle but rarely. Was going to replace coolant temperature sensor but not sure what else it could be, maybe have the bushing service done? any help is appreciated, thanks!

 

[ex-x-fire]

Make sure you check the throttle post. senser too. Vacuum leaks are common.

 

[vetteinaz]

Thank you still waiting to get the throttle boddies back to check, they were leaking vaccum bad hopping thats it.

 

[iroc37]

Thank you still waiting to get the throttle boddies back to check, they were leaking vaccum bad hopping thats it.

be sure to tune the tbi's. with a manometer. if you dont know how search the board. best thing i ever did to mine.

 

[GerryLP]

I agree with the earlier post. A good place to start looking is the rubber hoses. If not replaced for nearly 26 years, then systematically replace them. Start with the one attached to the thermo vacuum switch. More than likely you'll notice that other problems go away as well.

GerryLP :cool

 

[Hib Halverson]

When you first start the engine and for a period of time until when the ECM determines the engine is "warmed up", it runs in open loop with the fuel schedule straight from programming.

Once the engine warms, it goes into closed loop and, if it won't idle hot that can mean the ECM cannot control idle in closed loop.

There are many reasons for that, one being vacuum leaks. I'd check for those, first. Also, check for TPS adjustment, IAC condition and throttle bodies which are dirty.

 

[vetteinaz]

I'll have to try that, first I'm going to have to build a manometer and balance the throttle bodies. Thanks guys, I figure its good insurance so i'll just replace the vacuum lines. still waiting to get the throttle bodies back from being bushed, as soon as i do I'll post an update. I want to get it tuned and running!

 

[vetteinaz]

Ok got everything back the other day from being bushed and now installed on the car, it started right up no problems but my idle now has the opposite problem, it is sitting around the 1500 rpm range. I move the tps sensor with my finger to see if it would take the idle up and it does. any ideas on what this might be?
I am building the manometer right now so I will get the tb's balanced, just wanted to figure this out first. Also anyone have a vacuum diagram, I found coming from the vacuum canister a huge hose that goes to nothing right now?

 

[ex-x-fire]

You have to set the tps to a certain voltage at idle. You do this after the use of the manometer to balance the t-bodies, then set min idle (on rear t-body?), then adjust the tps. Google "setting idle & tps on crossfire corvette" Corvette Fever mag has an article on-line that is more detailed.

 

[vetteinaz]

thanks I got it idling great now balancing the throttle bodies was easy, one question tho which wires do you test for the tps sensor? the highest I get on a reading for the voltage on any of the wires I test is a 5.0?

 

[ex-x-fire]

I think you test the dark blue wire, take a DVOM, set it on the low volt scale, this that blue wire on the postive term. & ground the neg term. of the DVOM. The correct reading should be 0.525volt(+ or - 0.075v), so loosen the screws and adjust if it is out of range. At wot the volt at the dk blue wire will be over 4.00v. As you open the throttle the voltage should go up smoothly.

 

[vetteinaz]

Thanks guys, the car runs great now! I'm going to get some pics of it up soon, good investment at $3000 and only about $350 more in parts to fix!

 

[vetteinaz]

Alright now i seem to have the opposite problem, when the car is in open loop mode it doesn't want to stay idling, it will just flutter down and die or i have to double foot it to keep the rpm's up. when it goes into closed loop mode it runs fine and idles great. any ideas? this happens even after the car gets warmed up and I turn it off and back on again...thanks guys!