Stalling When Pulling Away From a Stop

[RichLo]

Would a faulty TPS result in only having issues when pulling away from a complete stop though? I feel like a faulty TPS would be a more persistent issue, including when on the freeway, no?

The TPS may not be completely faulty, possibly just worn where the two meet... if the throttle shaft spins before it engages the TPS, it wont give any more fuel.

 

[L31MaxExpress]

The TPS may not be completely faulty, possibly just worn where the two meet... if the throttle shaft spins before it engages the TPS, it wont give any more fuel.

I have driven TBI vehicles that have come in with a completely dead TPS a few times. The MAP gives a lot more of the burst of pump shot than the TPS. With a bad TPS, I have seen a high idle, a surging idle, lockup/shifting issues and felt a bit of a stumble but never had one stall out completely. It could happen, but never experienced that myself.

 

[RichLo]

Good to know L31, thanks. Just shooting out ideas here

 

[Machlen]

Yes sir, done it numerous times. When the EGR valve acts up, it goes to full EGR opening nearly the instant the EGR solenoid enables it. At low rpm it can cause a vicous stumble, surging or even stall.

Ok - hopefully this is the issue. I just want to nail down what it is and get this truck running well! To triple check - are you saying to plug off the vacuum inlet of the EGR valve after disconnecting it? Or plug off the vacuum line? Or plug both? Sorry, I’m new here…. Thanks as always for the help.

 

[Schurkey]

Your testing is flawed. It should not hold vacuum on the TBI era valve. The vacuum is bled off by a valve internal to the EGR to help control it.

Negative back-pressure EGR valves will hold vacuum and can be tested that way.

Positive back-pressure EGR valves will not hold vacuum so they aren't tested that way.

EGR valves that don't sense backpressure will hold vacuum like the Negative back-pressure valves.

My '88 5.7L EGR valve is a Negative backpressure style. I'm under the impression that all V6 and Small-block TBI EGR valves are Negative backpressure, indicated by the "N" stamped into the top of the valve near the part number (at least on the OEM valves.) I don't know about Big-Block TBI EGR valves. The service manual for the vehicle in question will confirm (may be in the "Emissions/Fuel" manual of the appropriate set.)

Keep in mind that verifying the vacuum diaphragm of the EGR valve is only one of about half-a-dozen different tests that need to be performed to verify the EGR system.

 

[Machlen]

Negative back-pressure EGR valves will hold vacuum and can be tested that way.

Positive back-pressure EGR valves will not hold vacuum so they aren't tested that way.

EGR valves that don't sense backpressure will hold vacuum like the Negative back-pressure valves.

My '88 5.7L EGR valve is a Negative backpressure style. I'm under the impression that all V6 and Small-block TBI EGR valves are Negative backpressure, indicated by the "N" stamped into the top of the valve near the part number (at least on the OEM valves.) I don't know about Big-Block TBI EGR valves. The service manual for the vehicle in question will confirm (may be in the "Emissions/Fuel" manual of the appropriate set.)

Keep in mind that verifying the vacuum diaphragm of the EGR valve is only one of about half-a-dozen different tests that need to be performed to verify the EGR system.

Thanks for the additional information. What do you think about the test mentioned in the above comment(s)? I’m going to give that a go tomorrow.

 

[GoToGuy]

You could always get the OE service manual, it has testing and diagnostic procedures.

 

[Schurkey]

Or plug off the vacuum line?

What do you think about the test mentioned in the above comment(s)?

The test above? Fast...but incomplete, and assumes that the EGR valve pintle is sealing properly--doesn't leak.

You need to read the service manual for EGR diagnosis.

 

[Machlen]

Yes sir, done it numerous times. When the EGR valve acts up, it goes to full EGR opening nearly the instant the EGR solenoid enables it. At low rpm it can cause a vicous stumble, surging or even stall.

Tried this - did not resolve the issue. HOWEVER; I have learned a bit more about the issue and even have learned how to avoid stalling.

When at a stop in drive: when pulling away from the stop with a natural/typical gentle press of the accelerator (easing into it) the truck will stall. BUT, if you lift off the brake and then press the accelerator as lightly as humanly possible, you can hear the RPMs hop up a bit and then you’re good to accelerate like normal.

So, at least I’ve found a way to reliably drive the truck. But, I’m still very curious what the issue is and would like to resolve it. Any ideas with this new information? (It’s not the TPS, I already replace it

 

[Schurkey]

You could always get the OE service manual, it has testing and diagnostic procedures.

THANK YOU! The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum.

did not resolve the issue.

CONNECT A SCAN TOOL, verify EVERY sensor and computer output.

When at a stop in drive: when pulling away from the stop with a natural/typical gentle press of the accelerator (easing into it) the truck will stall. BUT, if you lift off the brake and then press the accelerator as lightly as humanly possible, you can hear the RPMs hop up a bit and then you’re good to accelerate like normal.

CONNECT A SCAN TOOL, verify EVERY sensor and computer output.

I’m still very curious what the issue is and would like to resolve it. Any ideas with this new information?

CONNECT A SCAN TOOL, verify EVERY sensor and computer output.

Does this truck have an OEM EGR valve, or an aftermarket replacement? Some aftermarket replacement EGR valves fail to include a restriction in the vacuum nipple that eases/slows EGR valve opening. There's a wonderful photo of this somewhere on this site, but I'm too lazy to go find it.

(It’s not the TPS, I already replace it

You can replace the TPS a hundred times and not solve the problem if the problem is the wiring that connects the TPS to the computer; or a failed computer; or is the result of a failure of some other part not related to the TPS.

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Make sure the fuel filter is not plugging. New-to-you vehicle? Might as well replace the fuel filter, along with ALL the other fluids and filters except for A/C refrigerant/desiccant IF the A/C actually works.

2. Assure all "tune-up" parts and procedures are in good usable condition. Distributor cap, rotor, plug wires, spark plugs, PCV system, EGR system, EVAP system. Check initial spark timing, electronic spark advance, verify cranking compression, and look at the heated-air intake. Replace the O2 sensor. Avoid cheap-junk parts from Amazon and others with brand-names that look like a bunch of letters strung together at random.

Buying from a local parts store is more-likely to get you parts that AREN'T counterfeit.

3. Connect a scan tool, verify EVERY sensor, and computer output including IAC. Check fuel trims just before, and as the engine stalls.