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Category: Chevy
Satisfied Customers: 26032
Experience:  ASE certified tech ADVANCED LEVEL SPECIALIST. Wisconsin certified emissions state inspector
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1993 Chevy: TBI intermittently stumbles/stalls at idle..acceleration

Customer Question

1993 Chevy K1500 5.7L TBI intermittently stumbles/stalls at idle, and frequently falls flat on acceleration down the road. System is going lean, and after 20-30 seconds of running lean, then goes into open loop forever. I have pinned out the PCM grounds and checked circuit resistances (all good), All scan tool parameters are within proper operating ranges. Many of the sensors have been replaced to no avail, as has the fuel pump & injectors. Have a good injector spray pattern, excellent fuel pressure, and 02 still goes lean eventually. The system really is going lean, as I can richen it up with a shot of carb spray. I'm wondering if TPS somehow while reading good on scan tool is messing common signal return. If I pull the ECT connector we go full rich and the drivability issue is gone. I'm down to checking circuit resistances at PCM connector, but would sure like to short cut this process if anyone has some ideas.
Submitted: 6 years ago.
Category: Chevy
Expert:  AMEDEE replied 6 years ago.

Did you still need help?

Have you checked ignition timing or pulled codes from the computer?
Customer: replied 6 years ago.
Yes. Timing is set right on, and I have tried moving the timing to different than spec, but no change. Timing is steady and not jumping all over. It is a fuel issue. The only code that ever sets is a P044, lean exhaust, and the exhaust is really lean. We have swapped PCM/PROM with a same year Suburban, no change. Both vehicles run the same as this did using the other's PCM. These old speed density systems have some commong signal return circuits, and I'm thinking one of the sensors, perhaps the TPS, is somehow messing up the signal return, but within specs as no codes or sensors out of specs on scan tool.
Expert:  AMEDEE replied 6 years ago.
What is the fuel pressure at when this happens?

What is the o2 sensor voltage at when this happens?
Customer: replied 6 years ago.
Fuel pressure under all running conditions is always 12-13psi. 02 when vehicle is running lean drops to .5-1.5ish MV. Quite often the 02 seems to cool off on long hot idles (cats are removed), but as soon as throttle is moved system drops into closed loop. I have shot the 02 area with a temp gun and it is more than warm enough for the 02 to work properly 400F plus always on hot extended idle. The 02 will have a a number crosscounts lean to rich in closed loop, but as soon as I drive the trucvk down the road, or brake torque it up, it goes lean and drops into open loop. The PCM is commanding it lean for some reason, as I can disconnect the ECT and it goes full rich. Ican't find a vacuum leak anywhere, 17inches of steady vacuum hot idle. Truck engine does not have mechanical problems, as will idles and accelerate good when rich enough.
Expert:  AMEDEE replied 6 years ago.
And still no codes?

The o2 sensor voltage should not go above 1.0 volt.

What is the engine vacuum at when the engine is at 2500 rpm in park?
Customer: replied 6 years ago.

02 E does not go above 1V. The readings I gave you were MV. The problem is when the 02 goes lean .5-1.5ish MV, the PCM should call to richen the system up, but is does not. Long tern fuel trim seems to hang aroun 162-65 on a hot engine, short term fuel trim starts at 128 and sometimes goes to around 134.


Vacuum at 2500 in park is around 20inches.

Expert:  AMEDEE replied 6 years ago.
Ok.. Verify there is 18-20 of fuel pressure when you dead head the fuel pump. If low, verify there is good power and ground to the fuel pump and that the fuel filter is clear.
Also, make sure that the MAP sensor vacuum port in the throttle body is not restricted. This would make the engine run lean under a load as well.
Customer: replied 6 years ago.

I previously checked the MAP port for vacuum, OK. According to the specs for this truck, the fuel pressure should never go over 13psi (right off Mitchell). It is a brand new fuel pump, as previous has low pressure issue. Filter is not that old, and if plugged or if pressure regulator is malfunctioning, why do I never see fuel pressure at throttle body so much as waver lower than 12psi? Also, many times the lean condition will start just off idle so bad so that truck wil fall flat on its face and stall. There should not be a need for more than 13psi fuel at that low point in the RPM range. I was going to do a pump volume test, but really it should not be necessary as the pressure does not fall off.

Expert:  AMEDEE replied 6 years ago.
Maybe you have a bad fuel filter??

THe fuel pressure could be there but maybe not the volume...

Check the map sensor voltage under a load when this happens.
Customer: replied 6 years ago.
The MAP voltage is linear with engine RPM/load as it should be. MAP was replaced by previous owner as was a bunch of stuff. As I stated originally, I know it is a fuel going lean problem, just not seeing it on the gage. I'm going to hook up the fuel pressure gage again, and get it set up so I can drive it down the road and watch. What are you showing as fuel pressure specs on this vehicle at idle and at WOT?
Expert:  AMEDEE replied 6 years ago.
10 to 13 psi is the specs. However, be sure to pin off the return line to dead head the pump. It needs to reach 18 to 20 psi. This is very important!

Also, If the map sensor voltage coming back to the computer is off, this will cause the engine to run lean because the computer may not open the injectors up enough.

Also, the fuel injectors could just be bad too!
Customer: replied 6 years ago.

New fuel injectors rebuilt by a good company. Engine runs the same on these injectors as the old leaky ones. Spray pattern on these is clean, old injectors dripped enough to run after key shut off. Also these injectors came first of a good running boat, and injector company verified they would work on the truck.


MAP has been replaced, I have checked the MAP circuits, and MAP reads correctly on scan tool at every RPM. I have even disconnected MAP and other sensors to now avail.


I am sure all the sensor parameters are within their spec'ed ranges. I will try the pump dead head and pressure test.

Expert:  AMEDEE replied 6 years ago.
Sounds good!

Let me know!
Customer: replied 6 years ago.
Dead head fuel pressure is 15psi, and regulated fuel pressure is 12-13psi under all conditions. I completely tore the regulator apart, and all is good. I thought these systems were vacuum regulated internally, but they are not, so the dead head test does not apply to this system. The only way this system will ever have slightly different regulated pressure is depending where the truck is with regard to sea level (very slightly different with ambient temp). I rerouted the MAP to a different ported vacuum port, and it still reads the same on scan tool at all engine speeds, loaded and unloaded as it did on the MAP port. I replaced the fuel filter, cut it open found a couple water droplets (took fuel sample and let set, looks good), also put a can of dry gas in tank. Truck acts no different. Still eventually going lean then locks into open loop. I let the IAC adjust a good idle at about 60 counts, then unpluged, still leans out off idle and falls flat at times.
Expert:  AMEDEE replied 6 years ago.
WHat is the injector pulse width at when under a load when this happens?
Customer: replied 6 years ago.
Not sure because the truck seems to be running fine now. I guess I need to put some gas in it and take a run down the road. The other day I was watching the PW and at a hot idle it was 1.5-1.6 and then it seemed to fluctuate pretty quick between 2.1 - 3.5 when the truck was acting up. Even now it seems on the lean side, but will for a while have one 02 crosscount before the 20 second lean PCM clock expires throwing it into open loop. The 02 MV hot will frequently hang down around 56 MV to 156 MV and never cross over to rich for a long time.
Expert:  AMEDEE replied 6 years ago.
You did replace the o2 sensor...correct?
Customer: replied 6 years ago.
Yes, did that first days ago when it was setting lean code and would stop producing voltage. I knew that was not the whole issue because sometimes this thing runs terrible on a cold start, which is open loop. And on a cold start it takes a long time for the 02 to get hot enough to allow closed loop because the cats are missing. I verified that the truck really is running lean as indicated by the 02. While the truck is idling with 02 lean, a quick shot of carb spray in the throttle body will cause the 02 to go rich. And when the truck sometimes starts shaking at idle it will smooth out with carb spray. I jst don't understand why the computer is not trying to richen the mixture up based on the 02 saying lean.
Expert:  AMEDEE replied 6 years ago.
If you are confident that it is fuel and not ignition related, then the big three sensors besides the o2 sensor that will cause the engine to run lean is the map sensor (which has been replaced) engine coolant temp sensor and the tps sensor.
Customer: replied 6 years ago.
Yes, I know, and they have all been replaced recently. The TPS while a while ago, but the others very recently. Actually these speed density systems will run decent with just the MAP and TPS hooked up. I have already run the truck with everything disconnected, including the TPS. It just throws in a default TPS valuse when disconnected. I have pinned out most of the sensor circuits (reference, signal return, and grounds) and see nothng strange. If any of these circuits was way off, we would be getting codes, as the codes set when the sensors are disconnected (verified that). The current PCM & PROM are out of a different same year vehicle that ran perfect and runs perfect using this truck's PCM/PROM.
Expert:  AMEDEE replied 6 years ago.
Check the injector pulse width and the map voltage when this happens and let me know!
Customer: replied 6 years ago.
Another sort of strange thing is that the vehicle will go open loop on long hot idles (I thought could be the 02 cooling down, but is always 400F plus when checked), but as soon as the throttle is pushed the smallest amount it goes closed loop. Other times it stays open loop with throttle. I know after being lean for I believe 20 seconds strait the PCM goes open loop, and I think it then stays open loop until the ignition key is cycled. i'm about at the point to buy a performance chip that runs this thing open loop all the time and see how it runs then.
Customer: replied 6 years ago.
OK. Going for ride with fuel gage and scan tool connected.
Expert:  AMEDEE replied 6 years ago.
Customer: replied 6 years ago.
Went on a couple short runs and truck was not falling flat bad as usual, but still leans out and gets locked in open loop while running down the road. The 02 reading starts out at idle a little biased lean, but switching, then as soon as into accelerator truck stays lean until open loop. 58-65 mv is normal when leans out. Fuel gage never wavered at 12-13 psi. MAP around 2.9v when PW 2.8MS when truck seemed to be falling a little flat under brake torque. This thing is really close to getting a carburetor on it.
Expert:  AMEDEE replied 6 years ago.
Thanks for the info!

I am going to get some sleep. I will get back with you in the morning.

Some things to try would be to check exhaust back pressure and remove the vacuum hose to the EGR valve and see if that does the trick.

Just some ideas...
Customer: replied 6 years ago.
EGR vacuum is plugged off because EGR diaphram has a leak. I pulled EGR to inspect days ago, and pintle spring has good tension holding valve completely closed, not leaking at all. I know the EGR will richen mixture a little on a hot engine, but that is not the problem. I have worked on many of these trucks running around without cats, and with plugged off EGR's. The most that happens is under a load on a hot engine you might get a little spark knock because the PCM throws the timing out when the EGR is supposed to be open and leans the mixture a little. An open EGR midrange would help this issue, not hurt it. Only at idle will an open EGR create a vacuum leak stalling/rough engine operation. The short term fuel trim tonight was hanging around 132-135, which shows the PCM is adding a little fuel from base, but why is it not adding more. I believe on these truck the trim can go as high as 200.
Expert:  AMEDEE replied 6 years ago.
THanks for the info!

Maybe it would be better if I let other experts assist you with your question.

Good luck!
Customer: replied 6 years ago.
OK. Whatever will help. I just had the thing idling hot and let it lean out on the scanner, then unplugged ECT, 02 shot up to rich, engine light came on, and to my amazement stayed in closed loop. I shut the truck off and restarted with ECT still disconnected, it started in open loop then eventually went closed loop. ECT disconnected reads -40c, how are we going closed loop without ECT connected? It runs much better with ECT disconnected, and the 02 is cyling lean/rich as it should. I'm back to thinking one of the sensors on this common signal return is corrupting the return signal. ECT reads reasonable on scanner for engine temp, so not sure what is going on. Maybe I'll check the grounds again tomorrow on a hot engine. I'm done for tonight.

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