gwd1995Z28

I have a 1995 LT1. Intermittantly it seems to load-up and run rich diring the first mile under load between 2000-3500 rpm. Typically it starts and idles fine. Runs fine during at low rpm, but when pulling on to the highway with a load, up hill, it bogs down like it is getting too much fuel, even popping in the headers. If I punch the throttle and get it over 3500 rpms it revs to the shift point but bogs again after the shift. After about 1/4 mile the temp guage gets to normal and it clears up. It has only done this once hot. Used to do it the day or two after buying gas, but now fuel level is irrelevant. I have varied gas brands.

It seems to me that some sensor must be miss reading cold. My shop has been trying to figure this out for 2 weeks. MAP sensor voltage is a litle low but in range. They are thinking it gets condensation in the distributor over night. But in Colorado it is not all that humid. I don't really want to throw $700 at a distributor if there is a more likely solution. Any ideas?

gwd1995Z28

Additional info.. repair tech says there are not codes and all the sensors seem to have proper voltages both at the sensor and the ECM harness.

Injuneer

Sounds like something related to open loop vs closed loop operation. As soon as the coolant reaches normal temp, and the PCM can go into closed loop, the problem goes away. That would seem to point to a problem that needs the O2 sensors active in order to correct an excessively rich condition.

You mention the MAP sensor as a potential problem - What is your MAP sensor voltage? Better than that, what does the MAP sensor read at idle (in kPa or "Hg) and at WOT?

Provide additional info about the car. Appears you have headers, so its obviously not stock. The more info you provide about any engine modifications, the better chance you have getting help. Why not add a "signature" with basic info about the car - year, model, engine, tranny and any major mods. That way, the info will be appended to every post and no one will have to ask for additional info.

gwd1995Z28

1995 LT1 w/AC, Stock exhaust headers. modifications 1.6 GM lifters, 4 speed trans, chipped to move shift points to shift at 5300-5500 rpm borla 3" y pipe. MAP sensor voltage off 0.5 volt at 4.5v

Injuneer

So.... "stock exhaust headers" = stock exhaust "manifolds"?

Since you cant "chip" the trans, you must have used a hand-held programmer, a plug-in module, tuning software, or a mail-order tune to change the shift points in the PCM. What did you use?

Are you saying with the key on, engine off, the MAP sensor reads 4.5V? What is the elevation where you live? The MAP sensor reading (barometer) will be lower at higher elevations. 4.5V converts to about 92kPa, which would correspond to ~27"Hg, which would be normal at about 2,500-ft above sea level. Did you verify 5.0V reference reading across the gray and black wires?

gwd1995Z28

Stock exhuast manifolds. 7000 ft when MAP tested. Typical problems at 8300 ft but has happened as low as 5300 ft.

Shift program was done by prior owner so I don't know how it was done. It is just a bit or radical than the factory program. However, I doubt that is the issue since the problem started in March and the programing was done several years ago

Thanks for your interest and input.