94 GMC surges when it starts in 1st and 2nd

[94 Van Guy]

OK, 94 GMC 4.3 "W" engine, AWD with auto.

When I first start to drive and easy on the throttle, the van seems like I am stepping on the gas a little more then back easy. More throttle and the less. while all the time the peddle is at the same position. When it shifts into second the same thing. With a little more peddle, not a lot, same thing. When I give more throttle to get to 40-50 mph, I can't feel the problem. Cold engine or hot, same thing. It's almost like someone is advancing the distributor and back to normal. Like more power and then back.

I hope I have described the feeling I am getting. Also, today under a heavy load on the engine, getting to freeway speed with 3/4 throttle, I lost vacuum and the vents went to defrost, of foot, couldn't tell. When after I got to speed and backed off the throttle a little, the vents started to blow again. I fought with a vacuum leak, but thought it was fixed. After it cools, I will look under the hood and see if any vacuum hose is off or loose.

Any thoughts would be nice , and thanks, Keith

[Reno]

concerning the vents falling to defrost and floor, sounds like the check valve is on wrong or bad...maybe not there all together. since the engine isn't seeing much vac at all with the throttle open that much.

[94 Van Guy]

Thanks Reno,

I have just had the van to a shop I deal with two times and the dealer once. Replaced all vacuum lines except the ones in the dash and everything worked well for weeks. New check valve, the dealer found that the other shop replaced all the lines around the valve, but missed a $5 part.

The surging is what got me now.

Again, Thanks, Keith

[Astroplane]

It sure seems like a major vacuum leak. Possibly leaking internal to the crankcase. Any codes? Any detonation?

[94 Van Guy]

It sure seems like a major vacuum leak. Possibly leaking internal to the crankcase. Any codes? Any detonation?

Codes? The SES light only comes on before start up, the LAPS won't scan for codes unless the SES light comes on. I will get it scanned and see what comes up. and detonation?, not sure I understand. If you mean popping backwards, no, and starts on the first try, cold or hot, in about 1 second or less. I took it to a local shop for a vacuum leak, took it back, that time they replaced all but the check valve. Same problem, would loose vents under heavy throttle. I then took it to the dealer, they found a leak behind the battery, replaced the check valve and said nothing about codes. Maybe they didn't check.

Last night when I left the house, it seemed to be OK, but after it heated up, the surging was back. I will listen and be more attentive when I drive.

Thanks, Keith

[JaxSPL]

Quick and easy check. o2 sensor. The surging can be caused by a bad o2 sensor, believe it or not. Some of the symptoms you've explained are some of what I've been dealing with except I got a code 13 on the SES light.

IF you do have a bad o2 sensor, be careful. I just found out that my '89 sensors are $20, but a '93 is $85, at the same place. Took me for a loop when I found that out as there really isn't that much change.. I don't think.

[94 Van Guy]

Thanks for the tip, I need to get to the bottom of this. I only get 14.2 MPH and the power is much less than any other Astro/Safari I have owned. This is the first AWD and I thought that was the reason for the lack of power. Funny though, it starts easy and with exception of this issue, the truck has little wrong, I like this one the best. Just waiting for some snow to try the AWD.

I may just take it to an IM shop and see if I am running rich.

Keith

[94 Van Guy]

It sure seems like a major vacuum leak. Possibly leaking internal to the crankcase. Any codes? Any detonation?

If you speak if pre ignition spark knock, none.

[JaxSPL]

I am now in the same boat as you. Got my o2 sensor changed out today, reset the computer, and it's still doing the same thing, just not as bad.

[LEVE]

If you are going at a low speed, say 28mph, does the speed control hold steady?