350 TBI Runs Rough When EGR Valve Opens

[Nelson]

Hello, I'm new to the forum. I have a stock 1989 Chevy Suburban 350 TBI with an annoying problem. At low speeds, when the EGR valve opens, the truck runs rough and will almost stall. Its most noticeable if I'm pulling out of a parking lot where I'm driving at low speed with gentle acceleration. It idles great, but as soon as I touch the accelerator, the EGR valve opens and engine almost dies.

This doesn't occur when the engine is cold since the computer doesn't open the EGR valve when the engine is cold. The problem is most apparent when the engine first becomes warm. Once the engine is good and hot, the problem isn't quite as noticeable but is still present.

If I unhook the EGR valve solenoid or unplug and cap the vacuum line to the valve (leaving the EGR valve closed), the truck runs great. Eventually it will set a code and turn on the SES light, but otherwise, it runs fine. I've changed the EGR valve and solenoid several times (because I had them on hand) with no improvement.

I also monitored the ECM inputs with a laptop computer and verified that all of the sensors are giving the right outputs. TPS, vehicle speed, coolant temperature sensor, etc. are all sending the right signals to the ECM.

Plugs, wires, cap, rotor are fairly new (maybe 15,000 miles).

I suppose I could unhook the EGR valve and reprogram the ECM for EGR delete, but I'd rather find out why it isn't working the way GM intended. Since the car is stock, it should work properly with the EGR functioning.

It seems like the EGR circuit and valve are functioning properly, but for some reason, its causing a runability problem. I've read the excellent EGR write-up that's on this board and learned quite a bit about how EGR operates. Still can't figure it out. Any ideas?

 

[89Suburban]

Yeah, leave it unhooked. This is an age old question I gave up looking for the answer for.

 

[Camar068]

I've read the excellent EGR write-up that's on this board and learned quite a bit about how EGR operates. Still can't figure it out. Any ideas?

Kudo's on you reading up.

 

[Nelson]

Kudo's on you reading up.

Thanks! Guess I'll just unhook it. I'm not too worried about not having an EGR, I was mainly curious what might have changed (what broke or wore out) since it used to run quite well.

 

[Rusty Nail]

Buy a new EGR gasket.
Remove said EGR valve.
Verify vacuum line integrity.
Verify EGR valve freedom of movement and diaphragm integrity.
Using a shop vac and an ice pick or small flat head, clean the EGR port of the intake manifold.
Clean mounting surface.
Re-install using new gasket and report. Installation is the opposite of removal.

 

[Nelson]

I put the truck away for the winter, but did find a vacuum line that is getting a little brittle. When I get the truck out next Spring, I'll change the line and report back. I previously installed a new gasket, cleaned mating surfaces, and cleaned the EGR port as best as I could. Will let you know next Spring! I'd test it now, but I'd have to at least get it out and let it warm up before I could test it, and I'd rather wait until I put it back on the road in the Spring.

 

[Nelson]

Finally fixed it. After all this, it just needed a new EGR valve. I changed the EGR valve several times before, but always with used valves. I noticed that there was some difference between valves in how the truck ran, so I finally purchased a new valve. Now it runs perfectly!

I did change one vacuum line that was getting a little rough looking, but it was still holding vacuum.

 

[Cuba]

Great update! Love that you posted your findings... & good news to go with it.

 

[4WDKC]

Finally fixed it. After all this, it just needed a new EGR valve. I changed the EGR valve several times before, but always with used valves. I noticed that there was some difference between valves in how the truck ran, so I finally purchased a new valve. Now it runs perfectly!

I did change one vacuum line that was getting a little rough looking, but it was still holding vacuum.

I was going to suggest a vacuum leak, my truck was doing the same thing, i found a broken bolt for the exhaust manifold to the block causing a leak and making the truck run bad.

 

[Cuba]

I was going to suggest a vacuum leak, my truck was doing the same thing, i found a broken bolt for the exhaust manifold to the block causing a leak and making the truck run bad.

That's interesting. Didn't think leaks in the exhaust manifold would make that much difference. So you did this repair and there's a difference?

Besides noise & loss of backpressure... It didn't occur to me that there'd be a direct correlation with the way it runs. Obviously, there would b damage later on due to the extra heat though.

I have a customer with a newer Sierra but keeps procrastinating on repairing his broken exhaust manifold bolts. He had this notorious P0300 Random Misfire Code, with nothing else found. I told him that the exhaust manifold's condition is probably the cause.

 

[4WDKC]

That's interesting. Didn't think leaks in the exhaust manifold would make that much difference. So you did this repair and there's a difference?

Besides noise & loss of backpressure... It didn't occur to me that there'd be a direct correlation with the way it runs. Obviously, there would b damage later on due to the extra heat though.

I have a customer with a newer Sierra but keeps procrastinating on repairing his broken exhaust manifold bolts. He had this notorious P0300 Random Misfire Code, with nothing else found. I told him that the exhaust manifold's condition is probably the cause.

what it does is it draws fresh air in from the leak and blows across the o2 and tells it lean and the ecm will try to compensate, causing a rich condition and making the truck run poorly while still reading lean because of the fresh air. Have heard this is a common problem for 4x4 squares that the studs randomly break while driving.

 

[Cuba]

what it does is it draws fresh air in from the leak and blows across the o2 and tells it lean and the ecm will try to compensate, causing a rich condition and making the truck run poorly while still reading lean because of the fresh air. Have heard this is a common problem for 4x4 squares that the studs randomly break while driving.

Ahhh, nice tidbit of info. Makes sense. This issue is on a newer 4x2 GMC 5.3ltr... but also a common issue to have the manifold bolts break. 8 to 12 hr job to do both banks, but when he finally breaks down to do it, I'll be reinforcing the manifolds with steel brackets so this never happens again. lol smh. Thanks for teaching me something new!