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

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

Postby 94 Van Guy [OP] » July 27th 2010, 8:52pm

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 :confused: , and thanks, Keith


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

Postby Reno » July 28th 2010, 1:45am

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

Postby 94 Van Guy [OP] » July 28th 2010, 2:24am

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

Postby Astroplane » July 29th 2010, 9:18am

It sure seems like a major vacuum leak. Possibly leaking internal to the crankcase. Any codes? Any detonation?
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 29th 2010, 2:00pm

Astroplane wrote: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
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby JaxSPL » July 29th 2010, 4:18pm

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

Postby 94 Van Guy [OP] » July 29th 2010, 4:27pm

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.

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

Postby 94 Van Guy [OP] » July 29th 2010, 5:31pm

Astroplane wrote: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.
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby JaxSPL » July 29th 2010, 8:04pm

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

Postby LEVE » July 29th 2010, 8:23pm

If you are going at a low speed, say 28mph, does the speed control hold steady?
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 29th 2010, 9:04pm

Yes, If I set the cruse control it stays the same speed. With the peddle, it stays even, seems just to do it when you are accelerating until you get to speed. I can't lock the van into low gear but later today when I go out, I will try to see just how steady it really is in second.
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 29th 2010, 10:32pm

Seems I can't remember for beans. Just returned and got it to go into 1st gear. at 18-20 MPH, the surging is still there. when it gets to speed, it isn't as pronounced but still there.

Does this give any ideas what is happening?

Again, thanks for the help
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby LEVE » July 29th 2010, 10:45pm

Ok.. now try to isolate it and see if it does it in first gear only.
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 29th 2010, 10:51pm

Will do, but I must wait until I go out tonight.
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 30th 2010, 3:06am

OK, I went for a drive. First gear surges And the same in 2, 3 and 4th.. When I am driving in the freeway in 3rd. or 4th., I can feel the van doing the same as when it is in 1st.. It's harder to feel, but on a smooth patch of road at 55 or 65, you can still feel the surges. :confused:

Does this symptom help in a diagnosis? :shrug:

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

Postby Astroplane » July 30th 2010, 7:24am

Pull the PCV valve out of the grommet and use a large rubber cap or electrical tape to seal the valve intake. Drive it once and note any difference.
94 Van Guy wrote:I may just take it to an IM shop and see if I am running rich.

Pull a plug from each bank and inspect. What are the plugs trying to tell us?
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby LEVE » July 30th 2010, 2:56pm

OK, now you know it's surging all the time, not just between shift points. That mean's it's being told to serge by a sensor. I'd be putting a volt meter on the sensors (MAP, etc) and see what's happening with them.

Does the engine act the same stone-cold, or only serge when warmed up?
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 30th 2010, 3:50pm

Only noticed it when warm. I need to go get the battery re positioned. Seems Sears bolted the battery so far back, it is in the way for the shifting mechanism. I will be very attentive when it's cold and report back.

The PCV has not been replaced, to my knowledge, and will check what is going on. Mr. Astroplane, thanks for the tip about the PVC. In your first post, you referred to a large vacuum leak related to the crankcase, it went over my head. Not wrenching on autos for over 25 years, makes me forget some of the obvious. Mr. LEVE, I will check the Mass Air Flow sensor and others. One question, when I connect the DVM up, please explaine the process. Last time I did much work on autos, no such device existed.

It's getting fun being able to work on these vans, I won't even need to get grease on me, plastic gloves.(and I thought they were only for Doctors.) LOL

I'll keep yous informed, Keith
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby LEVE » July 30th 2010, 4:14pm

One question, when I connect the DVM up, please explaine the process.
You'll have to look at your manual to see the connections. It tells you to hook the DVM to a particular spot and monitor the voltage. There's a range for the voltage, too high or too low, and the sensor is bad. A MAF's a little harder as it's always changing state. But using a little creativity will go a long way.... like when the engine's stone clold.. using a hair dryer to heat up the element in the MAF and see if the state changes with the heat.
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Re: 94 GMC surges when it starts in 1st and 2nd

Postby 94 Van Guy [OP] » July 30th 2010, 5:21pm

LEVE wrote:You'll have to look at your manual to see the connections. It tells you to hook the DVM to a particular spot and monitor the voltage. There's a range for the voltage, too high or too low, and the sensor is bad. A MAF's a little harder as it's always changing state. But using a little creativity will go a long way.... like when the engine's stone clold.. using a hair dryer to heat up the element in the MAF and see if the state changes with the heat.



Thanks LEVE, I will have a look at the manual.

I went and tested the "problem," and when the engine is cold, there seemed to be more power, and no surging. After the van warmed up, the surging started. I stopped to get the battery adjusted, and when I started to drive away, in a big parking lot, the surging didn't start for about 30 seconds. When I merged onto the road, the surge was back.

Talked to my personal mechanic, the wife, and we will check the plugs and the voltage on the sensors, this weekend. It's like a team effort.
'91 Chevy G30 Bus 39.3K Miles


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94 Van Guy
250k+ Miles
250k+ Miles
Firing on 5 Cylinders (L3)
Firing on 5 Cylinders (L3)
Years of Membership: 94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years94 Van Guy has been a member for 11 full years
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Location: Salt Lake City, Utah USA
Gender: Male
Alias (AKA): Keith
Van Model Year: 1991
Van Make/Model: Chevrolet Astro
Extra Info: G30 Bus

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