Perhaps the EGR valve is sticking open when it's supposed to close when you let off the throttle..?..
CK5
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Help diagnosing, truck stalls
- Thread starter Houston_t
- Start date
Houston_t
Registered Member
How would i go about checking that ? This is one of the first i have ever bought that was still stock and had emissions parts on it.
Sometimes simply taking the vacuum hose off the EGR valve and plugging it will work for a test--provided a chunk of carbon under the EGR valve's pintle isn't what is keeping it from closing fully,in that case there will still be an internal vacuum leak ,it will suck in exhaust, and dilute the air/fuel mixture and lean it out enough to stall it..
If its sticking open due to carbon build up in the EGR port,you'll have to remove the valve and try to clean out the crud somehow so it can fully close..
I used to just take the EGR valve off and make a block off plate,but you probably cant delete one even temporarily on a computerized truck because the ECM controls when it's relays gets a vacuum signal to open it..it will know its been tampered with,set codes,light up the check engine light,etc..
Maybe hooking up a vacuum gauge to the intake (manifold vacuum) would let you see if the engine vacuum drops suddenly as you let off the throttle,which would indicate the EGR valve is either sticking open or isn't closing due to faulty EGR relays that control the vacuum to it..
It could be a few hundred other things too...this is why I'm reluctant to own a vehicle with a computer..it was tough enough to diagnose issues like this on a 40 year old truck..
If its sticking open due to carbon build up in the EGR port,you'll have to remove the valve and try to clean out the crud somehow so it can fully close..
I used to just take the EGR valve off and make a block off plate,but you probably cant delete one even temporarily on a computerized truck because the ECM controls when it's relays gets a vacuum signal to open it..it will know its been tampered with,set codes,light up the check engine light,etc..
Maybe hooking up a vacuum gauge to the intake (manifold vacuum) would let you see if the engine vacuum drops suddenly as you let off the throttle,which would indicate the EGR valve is either sticking open or isn't closing due to faulty EGR relays that control the vacuum to it..
It could be a few hundred other things too...this is why I'm reluctant to own a vehicle with a computer..it was tough enough to diagnose issues like this on a 40 year old truck..
rampage
3/4 ton status
Did you ever try to pull the codes? Remember, they stay stored for 50 starts. Even if the CEL only flashes for a second the code will stay stored, doesn’t need to be constantly lit.
Houston_t
Registered Member
Yes, only code i got was 34 and i read here that i will get that code if the truck stalls, wasn't sure if i needed to dig into it more.
Houston_t
Registered Member
I am going to vacuum test the EGR and clean it today after work.
rampage
3/4 ton status
Says here:
- 34. Map ( Manifold Air Pressure Sensor ) – Signal out of range, low resistance
I'd be a bit careful as there were some differences between the car and truck TBI systems, but this is a pretty good post on investigating code 34:
https://www.thirdgen.org/forums/tbi/423557-tbi-map-sensor-problem.html#post3344504
Reason I asked about VSS and cruise is that without VSS working properly, TBI setups will sometimes try and stall on deceleration. I haven't heard of that setting a code 34, but if you can download the service manual for your year here: http://67-72chevytrucks.com/vboard/showpost.php?p=5621043&postcount=1 it should tell you how code 34 can be set. It will also have a flowchart for trying to figure out why the code is being set.
Note that checking all wires and hoses is a first step, and the flowcharts always assume those checks have been done.
https://www.thirdgen.org/forums/tbi/423557-tbi-map-sensor-problem.html#post3344504
Reason I asked about VSS and cruise is that without VSS working properly, TBI setups will sometimes try and stall on deceleration. I haven't heard of that setting a code 34, but if you can download the service manual for your year here: http://67-72chevytrucks.com/vboard/showpost.php?p=5621043&postcount=1 it should tell you how code 34 can be set. It will also have a flowchart for trying to figure out why the code is being set.
Note that checking all wires and hoses is a first step, and the flowcharts always assume those checks have been done.
Houston_t
Registered Member
All vacuum lines look good, ERG valve seemed to be in excellent condition took it off, cleaned it and checked to see if it was holding a vacuum, then replaced gasket.
I still have not sprayed around yet to see if idle changes, or tested wiring with a mulit meter but those are the next on my to do list. Maybe today after work if i get time.
I still have not sprayed around yet to see if idle changes, or tested wiring with a mulit meter but those are the next on my to do list. Maybe today after work if i get time.
Power brake booster diaphragm can leak too,especially when the brakes are applied..had a Buick that wanted to stall when I stepped on the brakes years ago ...but I could hear hissing under the dash at the same time too,so it wasn't hard to diagnose..surprisingly it did not affect the power assist all that much either..
I'm not much help if its related to the ECM or sensors,etc,I haven't owned a new enough vehicle to get schooled in their issues..others here will be much more knowledgeable than I am on that subject..
I'm not much help if its related to the ECM or sensors,etc,I haven't owned a new enough vehicle to get schooled in their issues..others here will be much more knowledgeable than I am on that subject..
Houston_t
Registered Member
Houston_t
Registered Member
Latest update
Truck running MAP sensor readings
Pin A has a solid ground 0.1 ohms
Pin B has about .9 to 1 volt
Pin C has 5 volts
Sprayed carb cleaner around base of throttle body and rpm does not change.
This thing is so random, sometimes I can cold start it and it dies back out instantly but most times not. Sometimes I can drive 1 mile to the store and it stalls, other times I can drive all day with no issues. Mostly stalls when stopped but sometimes when moving, I’m at my breaking point.
Truck running MAP sensor readings
Pin A has a solid ground 0.1 ohms
Pin B has about .9 to 1 volt
Pin C has 5 volts
Sprayed carb cleaner around base of throttle body and rpm does not change.
This thing is so random, sometimes I can cold start it and it dies back out instantly but most times not. Sometimes I can drive 1 mile to the store and it stalls, other times I can drive all day with no issues. Mostly stalls when stopped but sometimes when moving, I’m at my breaking point.
Houston_t
Registered Member
After driving it today I noticed something
Whenever it stalls if I try to start it right back up it will not start. I have to turn the key to the off position and then back on and it will start. It’s almost like I have to key of for something to reset, this sound familiar to anyone ?
Whenever it stalls if I try to start it right back up it will not start. I have to turn the key to the off position and then back on and it will start. It’s almost like I have to key of for something to reset, this sound familiar to anyone ?
rampage
3/4 ton status
The fuel pump primes again after you restart, but you need to wait like 15 seconds for it to do that. Do you let it sit a few seconds or try to restart right away?
Houston_t
Registered Member
Try to restart right away, usually when this happens I’m driving it doing it while the truck is in motion a lot more now. It will die and I just slap it in neutral and restart it.
INChubbs29
Registered Member
I would check EGR, just fixed that on mine a few weeks ago, had the same issues of the low idle stall. After a good clean EGR valve and new gasket, night and day difference. My EGR was stuck open just enough to let in exhaust, and the IAC motor was staying wide open in an attempt to keep the engine running at idle. Also a clean egr doesn't mean that it is a healthy one, I have seen weak springs fail to close the valve correctly or close at all.
I would also look into the TPS sensor if you have one, a dead spot will be a big head ache, easy to test with a volt meter. Vacuum leaks are also a concern. IAC can come into question of course, and ignition coil, plug wires, cap and rotor never hurt to test.
I would also look into the TPS sensor if you have one, a dead spot will be a big head ache, easy to test with a volt meter. Vacuum leaks are also a concern. IAC can come into question of course, and ignition coil, plug wires, cap and rotor never hurt to test.
