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I have a 1995 Chevy Tahoe with a 350 tbi with 276000 miles that is running rough at idle and when I rev it. Things that have been replaced cap and rotor, plugs, plug wires, EGR, 02 sensor. manifold pressure is at 15 psi I have also checked compression 1-7 range 135 to 140 and one odd one at 160. I haven’t worked on these engines in a long time maybe you guys can help what should I look at next?
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Howdy and welcome.

Coolant temp sensor in the intake. Replace it because it’s probably original. Takes about two minutes if you’re quick on the draw. Pack some paper towels around the sensor, then have the new one ready. Deep well socket to loosen. Spin the old one out keeping pressure on it until you feel the threads start again. Quick swap and you’re done. If you’re fast you might lose a tablespoon or two. It’s a pipe thread so don’t go too crazy. Also, no thread tape.

Get your hands on a TBI fuel pressure test kit. Major chain parts store should have one to loan.
 

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Howdy and welcome.

Coolant temp sensor in the intake. Replace it because it’s probably original. Takes about two minutes if you’re quick on the draw. Pack some paper towels around the sensor, then have the new one ready. Deep well socket to loosen. Spin the old one out keeping pressure on it until you feel the threads start again. Quick swap and you’re done. If you’re fast you might lose a tablespoon or two. It’s a pipe thread so don’t go too crazy. Also, no thread tape.

Get your hands on a TBI fuel pressure test kit. Major chain parts store should have one to loan.
I will pick one up tonight and try to get it in. The fuel pump and filter are new put it in 3 days ago. thanks for helping.
 

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Discussion Starter #4
I replaced the coolant temp sensor no luck still running rough. Have any other suggestions?
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Still a believer in making sure a new pump is a good pump
 
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I had a failed delphi out of the box, completley failed in 13 months. Fukc I wish I did a pressure test. One day I couldn't accelerate like the pump was shutting off. the hesitation if you will was there from the beginning. Oddly I put the old ac delco back in, in a pinch. I also replaced the tps prior thinking it was the problem, I found a problem with the tps but it didnt fix the hesitation. If I only did a pressure test I could have seen it. Put the old pump back in, with a new tps, I spun the **** out of my tires like I never have before, lots of power with no hesitation. Problem solved and I still haven't done a pressure test. I'm guna put it on the xmas list for santa.

Al
 

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Discussion Starter #7
Ok did a fuel pressure test it is sitting at 13 psi stoped and running.
 

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was it running rough/the same before all those things were replaced or is this anew problem after replacement?

sounds like you know what you are doing, but might want to make sure you set timing correctly. There is a bypass connector you have to use - here is good link to troubleshooting, setting timing FAQ - FREQUENTLY ASKED QUESTIONS - PLEASE READ! - Chevy Message Forum - Restoration and Repair Help

some thoughts, quick things to check
-I’d check tps as it’s easy - make sure it’s right @ idle. Memory is 0.6v
-you probably checked for vacuum leaks, maybe disconnect, plug the vac to egr switch just to be sure that’s ok.
-probably unlikely but easy to unplug knock sensor and see if that changes anything
-look at injector spray pattern, see if any obvious problem there
[email protected] idle, it’d be interesting if it ever gets into closed loop (guessing not as something is preventing controlling fuel, timing correctly) but easy to check on ALDL- I forget how but you can google

I assume you checked fuel pressure after filter? If you replaced filter unlikely it has a pressure drop. I’ve heard of pressure regulator in TBI failing but that’s not quick to get into and check.

hopefully someone has more experience on what to check, good luck
 

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I was running rough before I changed all the parts. Let me tell you I just picked the Tahoe up for cheap.the person before me has replaced full exhaust from front to back with new cat and o2,water pump,cap and rotor, plugs, and the truck came with a new tank and fuel pump that I put in last weekend.I have replaced the coolant temp sensor and egr because it was bad.I have checked for vacuum leaks but can’t find any but that doesn’t say there isn’t one. I’m thinking I might have to do intake gasket theres a lot of oil on the intake I thought it was from a valve cover leak but it is just on the intake side.When I start it it idles at 900 and it revs fine and has no lights or codes. I will check the tps. Thank all of you for the help
 

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I can graph a tps on my cheap obd2 scanner. It should rise smoothly without any breaks or jumps whatsoever. Another source Of a vacume leak is at the vacume brake Booster. The grommet can leak but so can the actual diaphram itself. On my burb it was leaking but I had no idea as iv never experienced It before. When a vacume booster leaks, it sucks air from around the plunger that connects to the brake pedal itself. When it's bad you can hear it and your peddle will go to the floor and you wont be able to stop very well... its down right scary. Theres a way to test this with an obd2 scanner. You push the brakes and watch the short term and long term fuel trims. When you press the brake it operates the diaphram and a vacume leak will be sucked in from around the peddle plunger, if it's small you wont hear it but you can see the truck compensate for the vacume leak by throwing more fuel at it.... the short term or long term fuel trims (I cant actually remember which one) will rise up to 100% if you hold the brakes long enough and then when you let off and press the gas it will fall on its face and possibly stall out. Theres a great youtube video on this diagnosis and the entire replacement of the booster.

Good luck.

Al
 

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I can graph a tps on my cheap obd2 scanner. It should rise smoothly without any breaks or jumps whatsoever. Another source Of a vacume leak is at the vacume brake Booster. The grommet can leak but so can the actual diaphram itself. On my burb it was leaking but I had no idea as iv never experienced It before. When a vacume booster leaks, it sucks air from around the plunger that connects to the brake pedal itself. When it's bad you can hear it and your peddle will go to the floor and you wont be able to stop very well... its down right scary. Theres a way to test this with an obd2 scanner. You push the brakes and watch the short term and long term fuel trims. When you press the brake it operates the diaphram and a vacume leak will be sucked in from around the peddle plunger, if it's small you wont hear it but you can see the truck compensate for the vacume leak by throwing more fuel at it.... the short term or long term fuel trims (I cant actually remember which one) will rise up to 100% if you hold the brakes long enough and then when you let off and press the gas it will fall on its face and possibly stall out. Theres a great youtube video on this diagnosis and the entire replacement of the booster.

Good luck.

Al
The only problem is the 95 does not have obd2 it is obd1.
 

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Oh ya. Well damn I know there is a way to see that info but I'm not sure exactly how.

**** sorry

Al
 

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2004 Chevy Tahoe 2WD 5.3 Flex
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I am not a mechanic but I chased a similar problem for 2 years. Threw parts at it left and right. Plugs, wires, Fuel pump, regulator, filter, gaskets, rebuilt intake, smoke test, O2 sensors, MAF, MAP, CAM sensor, pressure tests good (wet and dry), on and on..

It ended up being a worn cam. Took me an afternoon to pull it out and inspect it. Free rental tools from Oriellys. It’s a fun project and gives you options. Reinstall if good, upgrade if bad, replace oil or water pumps while accessible but not required, etc... I wish I had done it much sooner..
 

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Problem solved the guy I got the truck off of did the cap rotor and wires I never checked them tell tonight and he had 5 and 7 wires crossed on the cap. Starts up and runs like a charm. thanks for all the help you guys are the bast.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Well, alright!

Glad it’s running better. I think about 90% of us here have crossed a couple plug wires before. I know I have.
 
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Problem solved the guy I got the truck off of did the cap rotor and wires I never checked them tell tonight and he had 5 and 7 wires crossed on the cap. Starts up and runs like a charm. thanks for all the help you guys are the bast.
Great to hear! Thanks for getting back with what it was.
 
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