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91 K1500 5.7 engine problem

91 K1500 5.7 engine problem

Postby kens_62 [OP] » Nov 28 2011, 4:01pm

I need help diagnosing an engine problem on a TBI 5.7 with 97K miles. It started a week ago when I felt the engine sputter a bit under load in OD. The problem progressed into "looping" at idle and finally stalling when accelerating from idle. Even with this, once you get to highway speed, the truck ran but felt like it was starved for fuel. It doesn't feel like and ignition problem since the truck never misses. But just to make sure I tried a new coil, new spark control module and new coil wire. Those didn't help.

I am thinking either a weak fuel pump or bad injectors. I have inspected the fuel lines and they seem solid.

If you've had this problem and have ideas, please let me know. I really don't want to replace the fuel pump if it isn't necessary.

Thanks!
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kens_62
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Related Topics  1997 Chevy K1500 Fuel system problem   1997 5.7 K1500. Engine dies after 1 hour running   Problem with engine temp and ac   2007 Silverado Engine problem   Tranny problem or engine sensor?   2009 SIERRA ALL TERRAIN AFM ENGINE PROBLEM   Engine Whining, power steering, brake and electrical problem   1988 k1500   98 K1500 5.7L Low Compression   i have a 99 k1500 with a 3 inch lift   1998 K1500 U-Joint   Spare Tire Tool for '96 K1500?   K1500 4x4 front diff rebuild   Misfire, backfireing, loosing power 5.7 K1500 
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Re: 91 K1500 5.7 engine problem

Postby filstoy » Nov 28 2011, 9:31pm

Did you do a fuel pressure test ? Should be 9-13 psi on your engine. I'd start there. If it's low I'd check/replace the fuel filter and test it again. Could be a fuel pump but not necessarily . Too expensive to guess. BTW, on my 1988 Silverado the fuel pump was easy to replace. I just unbolted the bed and slid it back enough to get at the pump. Didn't have to drop the tank.
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Re: 91 K1500 5.7 engine problem

Postby kens_62 [OP] » Nov 28 2011, 9:49pm

^ Yeah. I checked the vacuum thinking it could be a bad intake gasket. The vacuum seemed pretty steady around 17. I need to check the fuel pressure. I have the gauge, but according to my Haynes manual I will have to rig some type of tee into the supply line as well as a stop valve in the return. Not a lot of fun.

Very creative on slipping the bed back to get access to the tank! My truck is a long bed with a camper top so it wouldn't be quite as easy for me. *IF* it gets to that point, I will just take it to the local shop where they can put it one a lift.
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Re: 91 K1500 5.7 engine problem

Postby filstoy » Nov 29 2011, 1:11am

OTC makes that adapter already made up. You just take off your fuel filter and replace it with the adapter and hook up your pressure gauge to it. The number is OTC518487 . Most auto parts stores can order it for you or you can get it online . It's called a 'GM TBI Fuel Pressure Adapter'.
I didn't have a camper on the back of it but my truck is also a long bed regular cab. We only had to move it back 2 feet. My son and I just slid it back with me on one side and him on the other. Made it way easier because I couldn't even get a wrench up in there to loosen the lines. No friggin slack. A friend of mine that owns a shop gave me that trick. Had it done in an hour.
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Re: 91 K1500 5.7 engine problem

Postby kens_62 [OP] » Nov 29 2011, 4:19am

Thanks for the info on the adapter. I have one on the way! Never thought about connecting at the fuel filter. Good idea.

Actually, even with the camper top, sliding the bed 2 ft wouldn't be that had. I may give it a try if the pump is bad. :D
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Re: 91 K1500 5.7 engine problem

Postby kens_62 [OP] » Dec 05 2011, 6:44pm

Well, I spent the better part of the weekend working on this issue. I placed an adapter in the fuel line. It reads a steady 12psi so that rules out the fuel pump. I checked the EGR value and found it to be bad. I replaced it and the problem got slightly better. I tightened the intake bolts and the problem again seemed to improve, but still not close enough. Since I had ordered a set of intake gaskets, I decided to bite the bullet and replace them. I currently have the intake off and all mounting surfaces clean, but noticed that my set of AC Delco gaskets doesn't have ridge gaskets? My Haynes manual doesn't show them. Instead, it uses RTV gasket maker only on the ridges. Is this okay or should I go buy a cheap set of intake gaskets that has the ridge gaskets and use them with my AC Delco set?
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Re: 91 K1500 5.7 engine problem

Postby filstoy » Dec 05 2011, 9:07pm

By ridge do you mean the front and back end gaskets ? RTV works better then the front and back end gaskets as long as you give it enough time to cure. Also check your MAP sensor and IAC. The IAC is a little motor and the pintle gets suck if it's dirty. It doesn't always throw a code. If I remember right it's on the passenger side of your throttle body and is threaded in. They make a special Mass Air Flow cleaner that works good on it or you can just use Carb cleaner. The MAP sensor is a little trickier to check but your Haynes should tell you how to do it. You can probably look down the throttle body to see if the injectors are working. I'm also assuming you replaced or checked your fuel filter since you took it off to check the pressure.
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Re: 91 K1500 5.7 engine problem

Postby kens_62 [OP] » Dec 05 2011, 9:24pm

^ Yes. It is the front and end gaskets. I will go with RTV and give it time to cure. I will have to catch a warmer day since we are about to undergo a big weather change. Hopefully have everything ready to go this weekend.

I replaced the IAC. And yes, I replaced the fuel filter. I'm running out of ideas. The MAP sensor is one of the few things left I haven't check. The spray "looks" good from the injectors when I checked by pumping the throttle a bit. However, now that the throttle body is sitting on my bench, I've noticed some heavy deposits on the back (regulator) side that may indicate a leak???

On, one more clue. When I was road checking before tearing it down, the throttle response seemed much better, but I could feel a bit of pulsation when the engine was under load?

Thanks for all the help!
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Re: 91 K1500 5.7 engine problem

Postby kens_62 [OP] » Dec 19 2011, 3:23pm

Well, I'm still scratching my head on this one. I replaced the intake gaskets, rebuilt the TBI (I did not install new injectors). I installed a new map sensor. I tried a new ECU (using the old EPROM). Nothing is helping. I finally got timing back to about 2 degrees BTDC on idle and have the truck idling, but when I pull out, it just doesn't want to go. It isn't missing, rather it has very little power.

I am confused about the true ignition control module. Is it the unit located in the distributor OR the rectangular module mounted on the intake above #4 and #6 cylinders? If I unplug the rectangular unit, it doesn't effect the operation at all???? Is this normal?

Oh, one final thing. The engines idles much better with the 02 sensor unplugged. So, could a bad 02 sensor be causing this much grief. Could I have a bad ECU EPROM?
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Re: 91 K1500 5.7 engine problem

Postby filstoy » Dec 19 2011, 6:26pm

The ignition module is in the distributor. The rectangular unit you are talking about is probably the ESC module. Yes the EPROM's can go bad but when I worked for GM about 90 percent of the ECM's we got back were usually good. It was usually connectors, wires etc. I doubt your problems are caused by the Oxygen sensor but without live data I'm just guessing. Usually on that engine the gas mileage just goes down hill with a bad 02 sensor. Might want to take it in and get some diagnostics on it. Sounds like you've already replaced the usual culprits. It's all a computer system and it all works together so it can be very difficult to diagnose without a Tech 2 or something to see whats going on. A plugged Catalytic converter can also cause poor acceleration.
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