No Power, Won't Rev, NO CODE!!!!

[Garage Mechanic's Companion]

My '96 has me baffled again. This problem has been getting progressively worse over the past couple of months.

 

The truck starts and idles fine. Put it into gear and accelerate, you have to give it a significant amount of throttle to get it up to speed. If you put it to the floor, it won't rev beyond 4000 RPM. At ~2500 RPM with only enough throttle to maintain speed (transmission manually shifted to 3rd gear), it will surge and hunt. In OD at 55 mph (engine speed below 2000), it runs smoothly but has no power to run up hill or into a head wind.

 

IT WILL NOT THROW A CODE!!!!

 

As a shot in the dark, I changed the knock sensor (cause it was cheap), no difference.

 

Could it be the mass air flow sensor??

 

The truck has about 75000 miles on it.

 

 

HELP!!

 

Thanks in advance,

 

Derek

[MemphisDoug]

My '96 has me baffled again.  This problem has been getting progressively worse over the past couple of months.

 

The truck starts and idles fine.  Put it into gear and accelerate, you have to give it a significant amount of throttle to get it up to speed.  If you put it to the floor, it won't rev beyond 4000 RPM.  At ~2500 RPM with only enough throttle to maintain speed (transmission manually shifted to 3rd gear), it will surge and hunt.  In OD at 55 mph (engine speed below 2000), it runs smoothly but has no power to run up hill or into a head wind. 

 

IT WILL NOT THROW A CODE!!!!

 

As a shot in the dark, I changed the knock sensor (cause it was cheap), no difference.

 

Could it be the mass air flow sensor??

 

The truck has about 75000 miles on it.

 

 

HELP!!

 

Thanks in advance,

 

Derek

 

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Sounds like a bad fuel pump to me

[Garage Mechanic's Companion]

Sounds like a bad fuel pump to me

 

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I had that thought, too. But the pump comes on and shuts off again when you toggle the ignition without starting the engine. And wouldn't you get a code (i.e. low fuel pressure) with a bad pump?

[abqchevyman]

Try a new fuel filter. I have never had an electric fuel pump show signs of failure before it dies, they have always just died on me.

 

You might also want to check plugs and wires. It could be as simple as a shorted plug wire. When was it you last did a tune-up?

 

I would start with these before I started throwing $$ at it just trying to find it.

 

I suggest these because you mention that the problem has gotten worse, not a sudden issue. If it was sudden issue, then I would think about sensors etc. A progressive issue is normally tune-up related.

 

Good luck.

[Butch 02 Denali]

I would lean toward fuel filter, fuel pressure regulator or the pump. Check your fuel pressure with a gauge.

Still could be the mafs though like you thought. Try unpluging it and see if anything changes. Try giving it a good cleaning too.

Then check the vacuum line to the map sensor. If it's got vacuum then the map sensor itself may be bad.

[Garage Mechanic's Companion]

I would lean toward fuel filter, fuel pressure regulator or the pump. Check your fuel pressure with a gauge.

Still could be the mafs though like you thought. Try unpluging it and see if anything changes. Try giving it a good cleaning too.

Then check the vacuum line to the map sensor. If it's got vacuum then the map sensor itself may be bad.

 

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Thanks for the pointers, Butch. What would you use to clean the mafs? Any cleaner that should be avoided?

[Garage Mechanic's Companion]

abqchevyman:

 

The truck had a full tune-up ~5000 miles ago: New plugs, wires, cap, rotor, fuel filter, air filter... I'm all for changing the simple, cheap stuff first too.

 

I cleaned the MAFS tonight with some brake cleaner, no diff.

 

When I did a search, I found a case where an '03 Avalanche was suffering similar symptoms and it turned out to be a bad cat. Make any sense in my case? Any quick simple check?

 

Cheers,

 

Derek

[NONSTOP SPEED]

$50 says its your cat clogged up easy way to check unbolt the exhaust from the manifolds and see if it will run well other than being really loud if so its that or possibly your muffler but most likely the cat.

[Butch 02 Denali]

That's a good possibility too. They usually set a code, but you never know!

 

To check the cat, hook up a vacuum gauge to the intake manifold. Take a reading at idle. Lets say it's 20. Then take a reading at a steady 1500 rpm, then again at a steady 3000 rpm. If the vacuum drops more than a point or so at the higher rpm's then the cat is clogged. I'b say 1-2 ok, 2-4 partially clogged, and 5 or over definatly clogged and you found your problem!

 

Another way is to pull out an 02 sensor before the cat and hook up a pressure gauge to the 02 bung. Normal pressure is around 4-5lbs with a stock exhaust system. Do the same rpm tests as the vacuum gauge. Again, if the pressure rises dramatically at the higher rpm's the cat is clogged.

[Garage Mechanic's Companion]

Cool, I just happen to have a set of vacuum gauges. I'll hook it up and try it.

 

Thanks,

 

Derek

[Yukon04]

I had the same symptoms at about the same mileage on my last truck. It was teh cat clog issue. It did not throw any codes until it was REALLY bad. My money is on the cat's. Dealer covered under the 90,000 mile emmisions warranty.

[Garage Mechanic's Companion]

Checked with the dealer, cat warranty expired last year.

 

FIGURES.

 

Derek

[Garage Mechanic's Companion]

Okay, Here's what I know now:

 

At 1500 RPM, it puts up 20-21 in-hg of vacuum.

At 3000 RPM, it puts up ~18.5 in-hg of vacuum.

 

It will not rev above 4000 rpm sitting in park!! If I apply more throttle, the revs will actually drop to 3800, as if a load were being applied by a dyno, and the engine will start to ping.

 

When I went to shut the engine down after this test, the exhaust manifolds were starting to glow (the sun was going down and it was getting dark out, which made this easy to see). The exhaust that was coming out the tail pipe was hot to the touch and the truck had only been running for about 5 minutes.

 

What do you think?

 

BTW, I can get an after-market cat put on for about $315 CDN installed + gov't.

 

Derek

[Garage Mechanic's Companion]

 

[Butch 02 Denali]

That doesn't seem like enough vacuum drop to have a cat clogged bad enough so it won't rev over 4000!

 

The glowing hedders and the pinging are telling you its running out of fuel and going lean!

 

Check fuel pressure as you are revving it up. I bet it drops way off as the revs go up.

 

Thats way too much for a cat. You can get high flow magnaflow cats for around $50 US. A muffler shop should only charge about $50 to swap it!