1991 Silverado 350 TBI Cuts Out Under Acceleration

[mullint]

Hello everyone, I'm sorry this is long. I am have been having some trouble with my truck. As the title states it's a 1991 Silverado 350 TBI. The problem is under moderate to hard acceleration, the engine engine will hesitate or hold back to completely cutting out depending on how much the throttle is pressed. If pressed 3/4 or more the engine will cut out just like it shut off, then it will buck a few times, then pick up and rev up like normal. This only happens when the engine temp is between the 40-90 degree mark on the temp gauge. Once the truck reaches operating temp, it runs fine. I can take off with the throttle to the floor and it goes like it should. Also I should note, black smoke comes out the tail pipes after the bucking clears up.

The fuel pump was replaced because it actually did fail, at the same time I replaced the fuel filter, the TPS has been changed and while performing my second block, I scoped the reference and all was fine. I replaced the temp sensor, O2 sensor, I checked out the EGR, all to no change. I unhooked the MAP sensor and nothing changed there, I just didn't try testing it out with a another. I checked the fuel pressure regulator spring, all was good. Timing is dead on, myself and anyone I've talked to and demonstrated this to is at a lost. I've checked the distributor and there's no play in it, gave it new cap, rotors, wires and plugs.

Like I mentioned, this only occurs when the engine is between the 40-90 (just before the 100 mark), and it's every time. If I accelerate lightly, you can feel the engine loose power for a few seconds, then feel it come back. I would describe it a little worst then if you were driving a 4 cylinder and the A/C compressor engages. No DTC's present. Thanks.

[BigAC]

Did you change the temp sensor on the block or on the intake? If I remeber right the intake is just for the gauge and the one on the side of block is for the ecm. Also check all grounds. You really shouldnt drive it that hard when its that cold though. It causes a lot of engine wear.

[mullint]

I normally don't drive hard. I come across this a couple years ago accidently when someone was allowing me to turn. When I pressed the throttle down half way to make a quick turn, the truck just sat there and bucked. You could basically hear each cylinder firing. The problem is present under normal driving, just not to the severity that I described. It's like it's holding back momentarily under normal acceleration.

As for the coolant sensor, I replaced the one in the intake. That one appears to be for the ECM, as when I did unplug it, the engine ran horribly and extremely rich.

[BigAC]

Yes your right the one on the intake is for the ecm. I would check all your grounds next.

[mullint]

What grounds should I look at. I've thus far for grounds ran a 4 gauge wire from the engine block direct to where the battery grounds on the body. I cleaned the grounds in the junction section on the firewall. Could you please direct me to where other grounds should be checked? thanks

[BigAC]

That's all good stuff. Check/clean the one by the thermostat. That's the one for the ecm.

[NEUMANNZZ]

knock sensor hooked up?
timing advances when revved up?
when cold is idle high then settles when it warms up?

[mullint]

Knock sensor is hooked up, I did unhook it while trying different things out. Timing does advance when reved, mind you the engine was up to temp when I checked. Idle at cold is fast and does settle down as it warms up.

[NEUMANNZZ]

can you post results of a diagnostic scan or datalog when cold and fully warm?

[mullint]

Ok, I checked and moved the ground for the PCM from where it was to a good clean ground with no difference. As far as the scan, all I can tell you is that I get code "12", which is everything is ok.

[NEUMANNZZ]

what do you mean by performing my second block, I scoped the reference ?

post results.

[mullint]

I was doing my second block for my mechanics license, which I gave up on after getting laid off numerous times. While I was in the college I used a scope machine to check for any bad spots on the TPS 5V reference signal.

[NEUMANNZZ]

check fuel pressure.
ensure egr valve is working properly:
http://www.fullsizechevy.com/forum/g...eshooting.html

check for clogged cat.

[mullint]

Thanks I'm going to look into that over the weekend hopefully. Very informative.

[Sub_64]

My Brother is having a very simular problem and we are having trouble hunting down the problem. I'll keep track to see if you can find out what the problem is.

[mullint]

Sorry this is from my iPhone so spelling may be off. I've been busy lately so I have nit done much about this issue until recent. So far I've replaced the EGR to eliminate one issue with a new one from dealer. I have checked my tips again with a vantage scanner, tested good, no drop out when i used the scope function on it. I'm going to re-check my base timing soon. The scanner showed me that the engine is knocking terribly, and for some reason my BLM is about 122-124 while driving for the first 10 minutes.! When I stopped the BLM dropped to 108 at idle. I ran the engine for a minute at 2500RPM and te BLM only climbed to 111-112. Truck is going in for body work by then end of the month, so trouble shooting is gonna be put on hold in a couple weeks. I'm almost wanting to try another throttle body or at least a set of injectors from a known good engine.

[mullint]

I just noticed this morning that the flap in the breather cover assembly is staying closed long after the engine is up to temp. I'm talking a 20 minute drive with engine up to temp and idling for at least another 20 minutes. I'm going to unhook the spring and open it up to allow cooler air from the fender to enter more easily and see if that will at least help. I noticed this before, but never put any thought into it.

[Sierra383]

Just out of curosity what would happen if the knock sensor wasn't hooked up?

[mullint]

I never tried unhooking it while monitoring the sensors, however I did unhook it before and there was no change in driveability.

[NEUMANNZZ]

fuel pressure?