EGR Causing Failure to Idle?

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

SuperSpore

OBS Enthusiast
Joined
Oct 19, 2019
Messages
53
Reaction score
39
Location
Mars
Hello,
On a 91 c1500 7.4 Tbi - One day it just didn't want to idle. It kept dying. It had a previous miss or rough idle which I had chalked up to a bad plug wire connection, but nothing found.

You have to keep mashing the throttle to keep it on. It has since thrown a 43 code (knock sensor error).

Wondering if there's a simple way to test the egr valve by pulling the vacuum line?
I realize it also might be a bad alternator, but wanted to rule out the other simpler solutions.

thanks
 
Last edited:

skylark

I'm Awesome
Joined
Nov 15, 2011
Messages
7,839
Reaction score
7,658
Location
Grants Pass, OR
Not effectively. You can pull the valve and hold a vacuum on it to try to clean around the pintle and put it back on but it doesn't mean that you'll get all of the carbon out of it.
 

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,557
Reaction score
1,557
Location
Michigan
Code 43 is often due to someone cutting the knock sensor wire. You definitely want to check into that and it's usually fairly easy so long as the break/cut isn't right at the sensor harness.

When these things don't want to idle, it's usually the ICM. Been the case for me in two instances. If you unplug the ESC wire (black & tan, resides under the plastic cover on passenger side firewall) and it suddenly wants to idle perfect- there's your dinner.. You can swap that out or stab a whole new dizzy in if you fancy that.
 

SuperSpore

OBS Enthusiast
Joined
Oct 19, 2019
Messages
53
Reaction score
39
Location
Mars
Code 43 is often due to someone cutting the knock sensor wire. You definitely want to check into that and it's usually fairly easy so long as the break/cut isn't right at the sensor harness.

When these things don't want to idle, it's usually the ICM. Been the case for me in two instances. If you unplug the ESC wire (black & tan, resides under the plastic cover on passenger side firewall) and it suddenly wants to idle perfect- there's your dinner.. You can swap that out or stab a whole new dizzy in if you fancy that.

I took a look at the junction block, and all I find is a black and burgundy/red wire that resembles your description - you think that might be it?
Also, will the truck start without it connected, or does disconnection have to happen while it's running?
 

SuperSpore

OBS Enthusiast
Joined
Oct 19, 2019
Messages
53
Reaction score
39
Location
Mars
Update:
I tried disconnecting the timing advance wire to see if that might help (if it really is a knock sensor issue), and no change to hard start and failure to idle.


Question - in theory the egr should be closed when cold. So if it's hard to start and no idle when cold, it could be the valve is stuck wide open? if I block the egr valve and line going to selenoid, should that make it idle properly while cold?
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,970
Reaction score
13,755
Location
The Seasonally Frozen Wastelands
The EGR valve is spring-loaded closed. Takes vacuum to open it deliberately.

HOWEVER, you could have the valve STUCK open, or you could have the thing jammed open by a fleck of carbon or other debris. AND, it doesn't have to be WIDE open. Even a little leakage at idle will make the engine run bad.

A failed gasket could leak exhaust into the intake manifold without going through the EGR valve at all.

You won't know until you remove it and start examining stuff.





What is your fuel pressure?
What is the results of a cylinder-balance test?
What is the data stream showing when you connect a scan tool?
 

SuperSpore

OBS Enthusiast
Joined
Oct 19, 2019
Messages
53
Reaction score
39
Location
Mars
The EGR valve is spring-loaded closed. Takes vacuum to open it deliberately.

HOWEVER, you could have the valve STUCK open, or you could have the thing jammed open by a fleck of carbon or other debris. AND, it doesn't have to be WIDE open. Even a little leakage at idle will make the engine run bad.

A failed gasket could leak exhaust into the intake manifold without going through the EGR valve at all.

You won't know until you remove it and start examining stuff.





What is your fuel pressure?
What is the results of a cylinder-balance test?
What is the data stream showing when you connect a scan tool?


Unfortunately I don't have a scan tool to view data. To order a cable, download, learn the software, etc will take some time and I'm trying to trouble shoot what i can in the mean time, in a less than perfect situation.

I've removed the egr and the gasket is not bad. I was wondering if there is a process for blocking the egr tube that would reflect whether there was a spring/open issue.
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,970
Reaction score
13,755
Location
The Seasonally Frozen Wastelands
Install a blockoff plate with a fresh EGR gasket.

There's still the matter of cylinder balance test, and verifying fuel pressure.

NOT having a scan tool when working on these vehicles is a huge handicap. Time to buy or borrow one.
 

deckeda

OBS Enthusiast
Joined
Aug 1, 2018
Messages
55
Reaction score
18
Location
middle TN
Schurkey is there a scan tool you like and recommend for these early GMT400s?
 

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,557
Reaction score
1,557
Location
Michigan
Update:
I tried disconnecting the timing advance wire to see if that might help (if it really is a knock sensor issue), and no change to hard start and failure to idle.


Question - in theory the egr should be closed when cold. So if it's hard to start and no idle when cold, it could be the valve is stuck wide open? if I block the egr valve and line going to selenoid, should that make it idle properly while cold?
The black & tan one under that cover or..?

Like Shurkey said it should be closed at idle with no vacuum going to it. You can pull the vacuum line off to see if for some reason it is getting vacuum or not. And as he said there could be something sticking it open. But if when you pulled it off and you saw the pintle was seated correctly and closed, then you know that wasn't the issue. Next, check to see if the thing holds vacuum. It shouldn't take a whole lot to get it to start to open but it shouldn't bleed down either. If it does, replace the valve. Then once it or a replacement is in place, get the engine to run and then apply vacuum at idle with your vacuum pump. Engine should stumble and maybe die. That tells you at least your EGR port isn't plugged up and that the system should work properly. Now you've eliminated EGR as part of the problem. The rest you will need a scan tool for. I recommend a Fap-Off [Snap-On] MT2500.
 
Top