'87 Sierra EGR?

Great Site!!
My 125k mi. '87 GMC Shortbox 1500 350TBI 700R A.C. stumbles at cruising or at low throttle input(10-25%)

I've repaced cap,rotor,fuel filter and problem still occurs.(I'll do plugs next)
I've had the truck parked for a while , but the problem was starting before I parked it.

I did discover though when I disc. EGR the problem goes away.
If I roll the throttle up from idle the engine wants to die.

With my hand on the EGR , I can feel it moving , and when I diconnect it at that time , the engine comes back to smooth running.

The EGR valve seems to hold (blowing through it)
The EGR sol. seems to shut down on decel.

Am I close? or looking in the wrong area

Given the age and mileage of your truck I would definitely replace the EGR valve. While the diaphragm may move up and down, the pintle or valve portion of the EGR unit is probably erroded allowing exhaust gas to leak by at idle (when it shouldn't and causing a rough idle) and allowing too much exhaust gas into the intake stream when it does open (causing a loss of power and poor running). Yeah, you can run it without the EGR, but you'll find the engine will have a higher probability of engine knock...especially under load or high acceleration. The EGR tends to slow the combustion process by introducing inert (non-explosive) exhaust gas and thus reducing the probability of knock using 87 octane no lead.

Mike
98Sierra

Given the age and mileage of your truck I would definitely replace the EGR valve. While the diaphragm may move up and down, the pintle or valve portion of the EGR unit is probably erroded allowing exhaust gas to leak by at idle (when it shouldn't and causing a rough idle) and allowing too much exhaust gas into the intake stream when it does open (causing a loss of power and poor running). Yeah, you can run it without the EGR, but you'll find the engine will have a higher probability of engine knock...especially under load or high acceleration. The EGR tends to slow the combustion process by introducing inert (non-explosive) exhaust gas and thus reducing the probability of knock using 87 octane no lead.

Mike
98Sierra

I was thinking because it was holding when I blew through it , it's still good.

But , I guess I could just change it out.

I was thinking the EGR being unplugged and the engine smoothing out would be a indicator of something else.
I understand the excess exhaust into the intake would bog it down.....heck I'm just being gunshy at this point , and trying to think this one out.

Thanks for the input.

There may be something else....I'm not familiar with your particular emissions controls but typically the EGR valve is controlled by vacuum (in the pre-computer engines). When the engine is cold the EGR system does not function. Once the engine reaches a certain tempurature, the EGR valve will begin to operate. There should be some type of vacuum switching valve that is screwed into the block and able to sense coolant tempurature...it might have several vacuum lines plugged into it as it probably controls the distributor vacuum advance also.

If this were my truck, I would definitly replace the EGR valve (they are not that expensive...30 bucks maybe... and relatively easy to swap out). In addition to the erroding pintle, they can become carbon fouled which can keep the valve from seating correctly. I would replace the PCV valve and any crankcase breather filters (usually attached to the air cleaner). I would go through and replace *all* the vacuum lines with new ones as they are probably rotted, cracked and leaking; this will cause all sorts of problems. Another cause of grief on older smog engines is a plugged or restricted catalytic converter. If the cat is bad then it can also affect how the EGR system operates.....if it were my truck I'd replace it....besides, the old GM monolithic cats are very restrictive even when they were new. NAPA should be able to set you up with a replacement cat (this will be a little pricey....maybe a bit over a hundred bucks....check on their website napa.com or check the AutoZone website, autozone.com, also). I had to do this (and more) on an '85 S10 w/140K and once I was done, it ran like a top and passed its emissions check with flying colors.

It all just depends on how much money (is it worth the cost?) and what you have to do (pass emissions?) to get it on the road....

Good luck

Mike
98Sierra

There may be something else....I'm not familiar with your particular emissions controls but typically the EGR valve is controlled by vacuum (in the pre-computer engines). When the engine is cold the EGR system does not function. Once the engine reaches a certain tempurature, the EGR valve will begin to operate. There should be some type of vacuum switching valve that is screwed into the block and able to sense coolant tempurature...it might have several vacuum lines plugged into it as it probably controls the distributor vacuum advance also.

If this were my truck, I would definitly replace the EGR valve (they are not that expensive...30 bucks maybe... and relatively easy to swap out). In addition to the erroding pintle, they can become carbon fouled which can keep the valve from seating correctly. I would replace the PCV valve and any crankcase breather filters (usually attached to the air cleaner). I would go through and replace *all* the vacuum lines with new ones as they are probably rotted, cracked and leaking; this will cause all sorts of problems. Another cause of grief on older smog engines is a plugged or restricted catalytic converter. If the cat is bad then it can also affect how the EGR system operates.....if it were my truck I'd replace it....besides, the old GM monolithic cats are very restrictive even when they were new. NAPA should be able to set you up with a replacement cat (this will be a little pricey....maybe a bit over a hundred bucks....check on their website napa.com or check the AutoZone website, autozone.com, also). I had to do this (and more) on an '85 S10 w/140K and once I was done, it ran like a top and passed its emissions check with flying colors.

It all just depends on how much money (is it worth the cost?) and what you have to do (pass emissions?) to get it on the road....

Good luck

Mike
98Sierra

Yes emission HQ California.

The computer does control the EGR inb my case also.
(From what I've read )

Thankyou again for the response.....I want to get this truck on the road....it is fun to drive and it's just rotting away.
It has alot of good parts on it and I would like to bring it back to life...thanks again.:wink: