egr problems

[ewood01]

I have a 95 gmc k1500 5.7l. My problem is that at tip in throttle in park or in drive at around 800 to 900 rpms my egr & purge solenoid are commanded on at 99%. The truck surges and wants to almost die. I watched my scan tool and it showed desired & actual egr & purge are commanded to 99%. I checked vacuum and it is at around 18. I tapped into the vacuum for my map sensor and it shows vacuum dropping down to around 5 when you raise the throttle which is common on any motor. Only when my drops the egr comes on before it can recover. As soon as this happens the map sensor keeps seeing low vacuum while the egr is on and so it keeps the solenoid on. If I unhook vacuum to egr, the solenoid comes on and then goes right back off again as soon as the vacuum stabilizes again which is a matter of seconds. If I slowly bring the throttle up and there is no vacuum drop it doesnt do it. I tested the map and it is reading properly to the computer. It changes accordingly and I have swapped it with a known good map. The truck has a brand new gm crate motor in it with only 5000 miles on it. I put on a new y pipe with cat. New oxygen sensor. New ECT. New TPS. I tried a new egr solenoid. I checked the egr to make sure it worked. it passed a functional test. It pulls up like it is supposed to and I can see it going up when comanded to. It also will hold vacuum without bleeding off. I sprayed down for vacuum leaks and even tried resealing the intake just in case. I didnt have this problem before I put the new motor in. I have a two wire egr solenoid. And I am pretty sure it is a ported egr valve. If I leave the egr hooked up it makes the truck surge at low rpms and the egr doesnt go off the more gas I give. I double checked my base timing and even tried bumping it up to 2 degrees. Do the canister and egr solenoids share the same driver for ground. I believe they are control by a pulse width ground. and if so maybe my ground is bad in the computer. I have all my grounds hooked up to the motor. Two spots on the intake and one on the back of the head. I am at a loss at this point.

[mightybg]

from what I have read, the egr is controlled by 3 things;

1. temp .~25 c. (this enables the system)

2. MAP pressure

3. throttle position.

 

I am having a very similar problem ( if not exactly the same problem) on my 1991 k2500. Until i got computer software that told me the egr was opening to 80% ever time I hit the throttle, i had replaced everything on the engine with the exception of the temp. and map sensors (even the ecm). I will let you know if I finally get it fixed and what it took for me.

 

by the way, are you sure the EGR is supposed to be opening when vacuum is applied? the reason i ask is because some of the egrs use exhaust back pressure to actuate so they shouldn't move until sufficient backpressure is in the exhaust system (negative pressure egr). which you may already know. hope any of this helps.

[mightybg]

I just checked ALLDATA, i'm all football bat on that one. it should open with vacuum and engine off. the negative back pressure cause the valve to close when engine is running. sorry.

[mightybg]

I'll giv you a link for this cause I am confusing myself, 2 different types of backpressure egr's negative and positive. on one type you do need enough back pressure for the egr to move open and if the internal port that gets blocked of when backpressure exists is clogged the egr will open excessively and cause problems. because this port is internal it is diffucult to fix without replacing egr.

 

http://www.aa1car.com/library/egr.htm

[mightybg]

what are you vacuum reading when you put it in gear? or when you are travelling at high way speeeds?

 

i found mine at about 20" and when I put it in gear it dropped to about 18 and when i was travelling down the road it was dropping to 5 and below. after thinking it waws every thing else, yesterday i checked the timing chain. I thought that excessive egr travel was causing the vacuum drop, but found about 4 degrees of valve train retard due to worn out timing chain. I plan to replace it this weekend and will see how she runs.

[mightybg]

wasn't the problem, does anyone know wha the typical egr duty cycle should be?

[luthor]

My friend had the same problem with his GM EGR valves, it was fixed after he brought it to his dealer I did not get the exact detail how it was fixed but I heard that the dealer made some replacements on the parts... Goodluck though...

[ewood01]

Its a negative back pressure egr. But the egr valve doesnt appear to be the problem. It is functioning properly. Whether the egr valve is hooked up to the solenoid or not the computer tries to go full egr. I just put a gm crate motor in 7000 miles ago and this all started when I put the new motor in. I have double checked for vacuum leaks and didnt find any. I am going to check grounds at the computer tomorrow. It is possible I have feedback through one of my grounds and it is causing the comp to whack out. I checked to see if it is even trying to pulse width the egr but it doesnt. It just turns it full on. I tried a new computer. Didnt help. I am down to either a ground issue or a bad prom. I have talked to a couple gm techs with no luck. I tried three different scan tools to make sure what I was seeing was true. Genisys. Snap on brick and tech 1. No luck with any of them. Its a big grrrrr on this truck. I had a 20 year gm tech look at it and he did all the same things I did. So I felt pretty good that I am headed in the right direction.

[ewood01]

I did however get to hook up a scan tool to another 95 gmc pickup and off idle under same conditions his egr cam on but at a small percentage like around 30 to 35 and then as soon as you left off or after a brief period of time it went off again. But it never came on 99 percent. So i know there should be some egr there but not full. I do know for sure that the way it is supposed to work is the pulse width modulated solenoid that supplies vacuum to the egr is supposed to go on off on off and based on how quickly it does it dictates how much vacuum goes to the egr and this is how it achieves its percentages. So all it is doing is making and breaking the ground in the computer through a driver which powers the solenoid on and off. Only mine is just going full ground when it thinks it needs egr applied.