'87 Firebird 2.8 V6 stumbles & hesitates at operating temp

[SweetBearCub]

I'm here with a problem on my '87 Firebird 2.8 V6 that I need some help with.

First, some background information. I bought the car for $500 cash + TTTfrom a BHPH on the 1st of May 2007. The body is in near perfect condition (at least to an untrained 1st time GM buyer) and it runs very smooth, starting on the 1st turn of the key. The only problems I noted during my test drive were a non-functional odometer (Frozen at ~85,000 miles), inoperative A/C, minor coolant leakage onto passenger floor mat during heater use, (Leaking heater core?) and the interior trim needs a good bit of work. Otherwise, the car seemed to be an excellent way to get to & from work with a bit of style for cheap.

A couple of days ago, the car began to hesitate & stumble under load, and even died after cranking once. Revving it to a high RPM (3-5,000) is fine, and the problem only appears at RPMs under about 2500. When it is exhibiting the problem, the idle is uneven, although the car will not stall. It does seem to come very close though.

Further, the car will not exhibit the problem if it is cold. The problem kicks in once the car is at operating temperature. All fluid levels are OK and all gauges read in the normal middle ranges. The 'Service Engine Soon' light comes on briefly at engine start-up as a bulb check and goes out, as it should. It does not come on while the car is having this problem.

Do GM engine computers store faults codes for this type of problem without setting the SES light? If so, how can I read the codes stored in the system, and where can I get a list explaining the code meanings? Or better yet, can I perform a KOER test that might set a code for this so that I can get a better diagnosis? If yes, then how do I do this?

Thanks....

[hotrod_chevyz]

Sounds like it needs a heater core and a tune-up.

Start simple, Fuel/air/oil filters, oil and spark plug change, new plug wires ect. Do a visual inpection and see if any vac. hoses are misrouted or broken/cracked.

Its not good to breathe in the antifreeze vapors. You should at least bypass or replace the heater core.

Watch and make sure the oil pressure isnt dropping while its hot, and watch and make sure its not running too hot.

[SweetBearCub]

The tune-up will have to wait for a while, as I am very close to broke right now. However, I can check the vacuum lines. Where are the lines located, and is there a clear diagram which shows the proper routings? Also, how do I bypass the heater core?

Oh yes - The oil pressure settles just past 40 (PSI?) when the car is fully warmed up. The temp guage settles at about 210 to 220. (220 being middle range on the temp gauge, 40 being the middle range on the oil pressure gauge.)

Thanks!

[SweetBearCub]

Ok, I have some further diagnostic results for the tech wizards here.

I stopped by AutoZone and they let me borrow a tester for the car for a while. Using it, I pulled the codes - None except code 12, which is system pass. I then put it into 'Field Service Mode' and drove around a bit, observing the light. Whereas I had been getting a steady once per second flash while at idle in my driveway, indicating closed loop mode was active, as I drove around, it changed to flashing twice per second, indicating open loop mode, with intermittant pauses with the light on or off for extended periods, indicating that the system was too rich or too lean.

With that information in hand, where do I go from here?

Thanks...

[SweetBearCub]

I'm back with more info. While the car is doing the stumble dance at speed, or from a dead stop, if I floor it, the problem goes away for maybe 30 seconds. Hopefully someone can help me solve this problem soon!