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94 C3500 TBI 350 cutting out

tanman_2006

tanman_2006

Just a farm kid...
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The Ol Basket Case strikes again.

I just put a new fuel pump in at 112k, currently 120. Now my truck sputters until it dies if I go over 1/2-3/4 throttle. Floored for more than 30 seconds will bring me to a stop. Changed all filters and oil as well as spray out the throttle body since it was due and still no better.

Pulled fuel pump to make sure the clamps on the hose were tight and checked fuel psi. We are good still. With the vortec pump I am running over 20 psi wot. I also confirmed timing, sounds great until it hits the dead spot.

Am I due for the tps?
 
Get the pressure down. I read where others have found anything more than 18 psi on the low pressure injectors can cause issues with the pressure preventing the injectors from opening. For a stock truck you should have no more than 13 psi. Even modded they say 16 is the limit. Then go from there.
 
Runs 15psi to injectors solid but I checked the pressure preregulator at over 20 psi and doesn't drop. Just stating fuel supply isn't the issue
 
What is the pressure at the injectors? It should be no more than 13 for a stock engine. Tbi injectors are a peak and hold design IIRC, and can be held shut by excess fuel pressure. I highly doubt a tps would cause this issue, but if it's in question, unplug it and see if it runs better.
 
Ahh...the new -middle of the throttle fuel savings tps with enhanced wot fuel saver option.

Hopefully the new one gets it all cleared up.
 
WOT will only get to 4.5 volts if you're lucky. If you question it, unplug it and see if it stops it. The idle will be increased, and throttle response will be down, but it will run on just the map then.
 
It can cause a studder at the dead spot, but I highly doubt it will cause it to cut out. If it's going to 4.3-4.5 at wot, then thats all it needs. If you're getting that voltage and still cutting out, I would be looking elsewhere. Not saying it's not bad, but from my experience, bad tps's cause idle issues and hesitation when you stab the throttle. Injectors showing there age, injector drivers in the ecm failing from injectors shorting out in old age, bad map sensors, coolant temp sensor issues, and the #1 problem area in tbi's is the distributor. It's a very simple system, there just isn't much to fail, but if 1 piece of the puzzle messes up, it can wreak havoc. The Ignition control module is a KEY player in the system and a cause of alot of issues. You need to find out if it's fuel or spark, and go from there.
 
And my parts truck has an older throttle body on it with the different style sensor.

How else can I test things before ordering a sensor?
 
You can buy a serial cable and use one of the many free scanner programs to see what's what with it.
 
Knowing a dead spot in the tps, i would just out that sucker.

Something else- i got a call from a cousin of mine in Utah helping a friend with his 5.7 tbi not starting/ running right. Long story short (ya, we all know I can't do that) an intermediate connection at the fuel pump was messing with fuel pressure. Iirc there was an updated electrical connetcor made at the pump location. So hooking up your gauge and go for a drive with it in place is bot a bad idea.
 
Agree with the advice above. TPS is a wear item, is clearly shot, and isn't a wasted part regardless of cure or not.

Primitive antique ECM... Good luck getting the SOB to indicate anything wrong with a scanner. The antique ECM's like to "lie" and display table data when things are out of range. Open/closed loop is a clue to if it's telling a lie or not.

Does this HD application have an EGR? I have had a couple EPIC GM dealer couldn't find EGR failures. Failure mode is flopping wide open at some random point with vacuum applied rather than a controlled amount by exhaust backpressure. (Note a free flowing exhaust can cause more engine knock due to decreased EGR flow. EGR controls NOx and knock.) When they flop wide open just off idle and stall the engine outright it's easier to find. Disconnect vacuum line to EGR and see if the symptoms go away. If it's a random occurrence just replace the EGR. (It's the one part you can throw at an engine with symptoms like this without troubleshooting much.) Of course the Primitive Antique ECM is clueless while shooting fire out of the TBI from the engine running so badly. (Pop back.) (Even the electronic EGR's the ECM's are clueless about sticking open... Another EPIC took months to track down sticking open, stalling the engine at idle, and then closing failure.) http://www.tomco-inc.com/Tech_Tips/ttt26.pdf

Double check all the vacuum lines are where they are supposed to be. Especially the MAP sensor line. Verify it indeed has vacuum - I have seen the ports blocked solid from PCV carbon gunk buildup requiring removal of the TBI to clean out. Mainly working on it due to oil leaks and was "taught" about the PCV's role in reducing oil leaks. (PCV was plugged solid in the TBI.)

Lets say your injectors do open at the higher pressure. This causes the engine to run rich as the calibration is for lower pressure. Carbon fouled spark plugs and a melted and clogged cat are possible. Fouled plugs quit firing and dump fuel into the cat melting it down. It's also possible you are flooding the engine at higher throttle: does it smoke black when it quits running at higher throttle?

Flip side if the injectors stick closed instant lean. This lean or no fuel condition makes the High Voltage ignition fire somewhere else besides the spark plug gap. Cap, rotor, or light the spark plug wires up like a Christmas Tree as it blows holes in them. Spark plug wires can go 100K or be burnt up by a lean condition. The coil wire is generally the first to reliably get a high voltage short. Snap the throttle WOT in the dark looking at the wires under the hood - a spark show means you are going lean and toasted the wires. Wires are a symptom in this example: lean is the issue. (Going rich is easy to fire so no HV ignition breakdown.) Note the coil can also short, but, generally the wires burn through first.
 
Last edited:
WarWagon said:
Agree with the advice above. TPS is a wear item, is clearly shot, and isn't a wasted part regardless of cure or not.

Primitive antique ECM... Good luck getting the SOB to indicate anything wrong with a scanner. The antique ECM's like to "lie" and display table data when things are out of range. Open/closed loop is a clue to if it's telling a lie or not. His 94 could have a basic ecm, or it could have the one that is the benchmark that the aftermarket used for there designs, and is still used today by a few. It could also be obd2 capable, and well regarded as highly capable. As to the data, GM used a basic data stream for tbi trucks, it doesn't tell you where to look, merely gives you raw data to view. The trims are the most important data feed, and most don't know how to interpret them, or what they mean. I don't do enough of them anymore to keep up with what they indicated as they don't read like modern fuel trims.

Does this HD application have an EGR? I have had a couple EPIC GM dealer couldn't find EGR failures. Failure mode is flopping wide open at some random point with vacuum applied rather than a controlled amount by exhaust backpressure. (Note a free flowing exhaust can cause more engine knock due to decreased EGR flow. EGR controls NOx and knock.) When they flop wide open just off idle and stall the engine outright it's easier to find. Disconnect vacuum line to EGR and see if the symptoms go away. If it's a random occurrence just replace the EGR. (It's the one part you can throw at an engine with symptoms like this without troubleshooting much.) Of course the Primitive Antique ECM is clueless while shooting fire out of the TBI from the engine running so badly. (Pop back.) (Even the electronic EGR's the ECM's are clueless about sticking open... Another EPIC took months to track down sticking open, stalling the engine at idle, and then closing failure.) http://www.tomco-inc.com/Tech_Tips/ttt26.pdf ALL GM TBI pickups I have came with an egr valve(even the SUPER rare 96 G van vortec TBI came with one). It is not controlled by exhaust back pressure. It works via a ported vacuum port in the tbi unit. Theres a solenoid to inhibit operation when the engine is cold, but once it reaches around 140-160 degrees, the ECM turns the EGR solenoid on when the engine is under a steady load situation, and if the throttle is open far enough, vacuum is sent directly to them to go full open through a metered orifice. They're not that cheap anymore either, got one sitting here waiting to go in the neighbors truck once he gets his engine swap done.

Double check all the vacuum lines are where they are supposed to be. Especially the MAP sensor line. Verify it indeed has vacuum - I have seen the ports blocked solid from PCV carbon gunk buildup requiring removal of the TBI to clean out. Mainly working on it due to oil leaks and was "taught" about the PCV's role in reducing oil leaks. (PCV was plugged solid in the TBI.) Fortunately in stock form there is only a couple of vacuum lines. The large 1/4" one off the rear of the tbi to the map, the egr off the front of the tbi passenger side, pretty sure by 94 GM had done away with all the air pumps, so the front drivers side port is not there, the front center port that runs over to the vapor canister, the power brake hose via a fitting in the intake, and I believe the pcv is also via a fitting in the intake. Then theres the crankcase vent line that comes over to the spacer on top of the tbi to air filter.

Lets say your injectors do open at the higher pressure. This causes the engine to run rich as the calibration is for lower pressure. Carbon fouled spark plugs and a melted and clogged cat are possible. Fouled plugs quit firing and dump fuel into the cat melting it down. It's also possible you are flooding the engine at higher throttle: does it smoke black when it quits running at higher throttle? To a point. Tbi injectors were designed to work and deliver steady fuel flow across a fairly wide pressure range. Stock pressure ranges from 8-13 psi with 11-12 being optimal and where GM set the regulators new. Too much pressure can actually cause the injectors not to open though. These are a peak and hold design where they are feed a high amp impulse then held open with a lower current. So increasing the pressure to much can actually make it so the initial impulse won't open them. This is why I advised against using a vortec fuel pump in it.

Flip side if the injectors stick closed instant lean. This lean or no fuel condition makes the High Voltage ignition fire somewhere else besides the spark plug gap. Cap, rotor, or light the spark plug wires up like a Christmas Tree as it blows holes in them. Spark plug wires can go 100K or be burnt up by a lean condition. The coil wire is generally the first to reliably get a high voltage short. Snap the throttle WOT in the dark looking at the wires under the hood - a spark show means you are going lean and toasted the wires. Wires are a symptom in this example: lean is the issue. (Going rich is easy to fire so no HV ignition breakdown.) Note the coil can also short, but, generally the wires burn through first.
Click to expand...

Added to the info.
 
Both EGR nightmares were CA emissions vehicles so frankly all bets are off as to what they used. The ECM's were clueless that the EGR's were stuck open even with GM Tech I and Snap On scanners of the day. Main reason for EGR discussion is symptoms match my past PIA to hunt down random problems. The "backpressure" control I describe is in depth in the link below. At the end of the day read the part number off YOUR EGR valve paying attention to "P" or "N" stamped on it. Model year and engine is NOT enough information for an EGR valve!!! (Sometimes even the VIN is useless.)

http://www.gmsquarebody.com/forum/showthread.php?t=6902

Per the link above apparently I had the 1993 MY 4.3L (ST). (I still hate the memory of that Fing gutless lemon over Patch even though I have thrown more engines at Patch. Patch has 4x the age, 2.5x the miles, and stock brakes that are worth a damn.) The 1993 1/2 ton 2WD CA Emissions 4.3L TBI (160HP) pickup I had used the following Electronic EGR, EVRV solenoid, shown in the how to clean video. My first clue was stalling coming off the freeway when the valve stuck. High RPM would keep the engine running and 2 blocks later at the GM dealer it managed to close itself - NPF. Weeks later I removed it when the engine was running bad and caught the dammed thing stuck open. The out of state dealer in Colorado couldn't find it, even by VIN, until we looked at 1994 MY as all they could find were vacuum operated EGR's. It cost 2x what the vac operated unit did. As I recall 1994's were the 1st year of the 4.3 Vortec (200HP) in pickups. First I ever seen the bleeping thing used elsewhere for 1993:

 
Under 8600 gvwr had back pressure referenced egr's, over 8600 like his 3500 does not. You're comparing 2 completely different emission control system's.
 
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