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Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4508
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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1995 k2500 5.7 engine, I have installed new plug.fuel..rotor.

Resolved Question:

I have a 1995 K2500 5.7 engine, I have installed new plug.fuel pump.filter.cap. rotor. distributer. new wires.and a holley throttle body, It still runs rough and get 10 to 11 MPG
Submitted: 5 years ago.
Category: Chevy
Expert:  Jerry Newton replied 5 years ago.
How is the compression on this engine, have you tested it with a compression tester?
Customer: replied 5 years ago.
I have a crate engine, transmission, from GMC it has approximate 80k
Expert:  Jerry Newton replied 5 years ago.
OK. That doesn't make it good, and I hate to assume anything, but for the sake of argument, let's say that compression is good across the board.

Have you checked/adjusted the ignition timing?
Customer: replied 5 years ago.
Expert:  Jerry Newton replied 5 years ago.
And you adjusted the timing using the procedure on the label under the hood?
Customer: replied 5 years ago.
It has in Keathing Chevy deal shop twice and they have installed a new chip and did everthing except check the compresson on each cyclinder, it used no oil 1/2 QT between changes
Expert:  Jerry Newton replied 5 years ago.
OK. I don't mean to belabor the ignition timing point, but that is one thing that is commonly overlooked or performed incorrectly, and has a dramatic effect on fuel economy and performance, so I wanted to make sure.

When you say it runs rough, do you mean at idle, when accelerating, at highway speeds..... what's going on there?
Customer: replied 5 years ago.
at a idle, runs fair on the road, has no power if your on the deer lease and get into a small hole, have to use the 4-wheel drive.
Expert:  Jerry Newton replied 5 years ago.
The things that I would do if your truck were in my shop:

1: Test the compression in all cylinder, make sure they're all the same and sufficient (130psi or better).

2: Test the exhaust back pressure with a gauge, to make sure the exhaust isn't restricted. Should be less than 1psi back pressure, a gauge made for this purpose is the only way to know for sure.

3: Double check the ignition timing, I would want to know that it's dead on. Unfortunately, just because it's been at the dealer doesn't mean a qualified mechanic worked on it. Chevy engines haven't had adjustable timing since 1995, so if you're dealing with a mechanic that doesn't have 15-20 years of experience, they may not know what they're doing here. I'm not badmouthing; I would just check it myself to be sure.

4: Check the timing chain free play. You can do this by putting a wrench on the crankshaft and moving it back and forth, while watching the distributor rotor for movement. If the chain has worn or the gear has come apart, it will skip cam timing, and cause dramatic power and MPG problems.

5: Check the knock sensor for correct operation. The knock sensor can take the timing advance right out of the engine, and again, cause dramatic power loss and MPG problems.

Those are things I would check right out of the box. I've seen over-sensitive knock sensors that cause this problem, it's worth looking at the scan tool parameter to see if the PCM is picking up knock activity.
Customer: replied 5 years ago.
It has a new exhaust system, true on the knowledge, I have had it in three shops, Miss, North Carl. and in Texas So I am at my wits end.
Expert:  Jerry Newton replied 5 years ago.
I understand.

I've been sitting here kind of thinking about it a little more. MAP sensor vacuum hoses are problematic, they tend to split/leak, and the port gets clogged up in the throttle body, but I'm sure that if you have a new throttle body, this isn't a problem for you.

Does it have a new catalytic converter? Does it ever have a service engine soon light?
Customer: replied 5 years ago.
Yes, no service engine light
Expert:  Jerry Newton replied 5 years ago.
The knock sensor problem I referenced above was a weird one, took me awhile to figure that out. That was on the same engine as your truck. No codes, just had to watch the scan tool to see what was happening as I drove. Knock sensor activity was going crazy on that one, although there was no audible knocking going on. I ended up doing some fancy wiring modification to that one to make it better, after exhausting everything else.

I don't know that this is what's wrong with your truck, but it's a good example of using the scan tool for more than just pulling codes. The scan tool data tells the mechanic everything that's going on in real time, it shouldn't be too hard to figure out where the power is going on this engine.

Unfortunately, I just can't do it from here. I love these sorts of challenges, but I haven't figured out a way to repair those things over the internet. Yet.
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