I need some help diagnosing my truck

the vehicle
1994 C1500
5.7L engine
Auto 4l60e trans

I need some help diagnosing my truck. I was driving up a long hill today on the highway. I would step on the gas fairly hard to accelerate quick and all I get is sputtering and I actually heard clanking out my back window when it was sputtering. Its been doing this for a while but I never heard the clanking before. It might not even be related because it could have been my exhaust hitting something from all of the vibration. I've also been noticing that my transmission doesn't kick down nearly as quickly as it used to.
At first I was thinking that it could be the map sensor or something vacuum related. I'm not sure anymore. Hopefully someone can point me in the right direction. Thanks

have u checked your plugs to see if you are running very lean? if this happens every time u step on the gas, i mean 100%, then whatever it is is either completely broke and would throw a code or its a mechanical issue. if under heavy acceleration only it does this, question the 3 things it takes to make your engine run. first, is it getting enough air? did a squirrel crawl into your intake and die or something lol. second, is it getting enough fuel? the spark plugs should reflect that at least to a degree. if u know u replaced the fuel filter recently and u dont doubt the fuel pump, are the injectors dirty? third, does it get enough spark? no missing so your wires are good, ur plugs may not be gapped correctly, but i doubt it. maybe rotor is broke in some way to where it advances or retards timing more than the computer wants it to under a heavy load. if u could give a bit more detail it would help a bit in diagnosing.

Have you changed your fuel filter lately?

maybe the clank is backfiring in the exhaust due to raw fuel from a cylinder or two not firing. a buddy of mine had all his plug wires melt onto his exhaust manifold on one side and his little 305 sounded like it had a racing cam because of it.

The clanking did almost seem like a backfire. I kept doing it while I was going up the hill and the people behind me were looking at me weird at the end of the exit. I was probably shooting flames out the exhaust, and they probably thought I was doing it on purpose (lol).

- I did change my fuel filter about 4,000 miles ago.
- The air filter is good (K+N just cleaned).
- Not sure If its getting enough fuel. I haven't ruled out the fuel pump.
- I checked two spark plugs today and had to stop there because one broke inside the head. What a pain in the _ to get out. I guess I have to get some spark plugs now. But to answer the question, they don't look to lean(not 100% positive). They do kinda look like crap though for only being about 30,000 miles old. The electrode was wearing on one side.
- The injectors should definitely not be dirty. I just rebuilt the throttle body. The spray looks fine and they don't leak.
- I just replaced the rotor in the distributor and cleaned up all of the electrical contacts in the cap

It does only occur under heavy load. Uphill or when I'm accelerating hard.

It does only occur under heavy load. Uphill or when I'm accelerating hard.

If you have a vacuum pump, you should check to see if the MAP is holding a vacuum. The MAPs job is to inform the ECM when the engine is under load so that the ECM can add fuel. Also verify that the vacuum hose to the MAP is not cracked or broken.

If is holds a vacuum, you can also use the pump to verify that it is returning the correct values to the ECM. I don't have the chart in front of me but I am sure someone can post it if you need it.

Low fuel pressure can also be an issue under load since the injectors can not delivery enough fuel when the pressure is low.

Unfortunately, you need a special fuel pressure tester for TBI. Pressure should be 9 - 13 PSI.

//2000CAYukon

spray starter fluid, start truck, spray around areas vacuum could leak, if idle is affected, there is your leak. if u rebuilt the TB, check the gasket and torque of those nuts. a tiny vacuum leak is huge under load.

I got some good info today. I got access to a GM Tech 2 scanner. I took a ride with it hooked up and kept getting it to sputter. A couple things caught my eye. It ended up with 5 knock counts over a ten minute interval. When it started sputtering I noticed the timing was going down to almost 0 from about 20 degrees (or a little over 20). I also did this with the scanner monitoring the transmission and was getting some 3-2 shift fault. I can't remember what the fault was for, it was a 3-2 something. The sputtering does seem to happen in between the 3-2 downshifts. Let me know what you guys think. I am thinking knock sensor. But I can't get any codes out of it to be 100% sure.

Have you checked the cat converter. Not sure if your o2 sensor is in front of, or behind, but what you are describing sounds like a bad cat. The reason I mention the o2 sensor is due to the timing fluctuation you saw.

I modified the cat a long time ago. Nothing catalytic about it at this point.
The oxygen sensor was replaced a short time ago. It could always be faulty, but I'm assuming not. It is ahead of the cat by the way

I've had similar issues with a plugged fuel filter or a clogged fuel sock in the tank back when shell had the fuel problems plugging up the modules and socks.. Check the fuel pressure and flow.. It should be 9-14psi and pump about a quarter pop bottle on initialization... The tranny won't shift right if the engine isn't working right.. everything works in sync

Somebody told me to change the plug wires and plugs. I'm a little skeptical that this will solve my problem.

"change the plugs and wires" is a pretty common thing to say...

[QUOTE=prankstar]The clanking did almost seem like a backfire. I kept doing it while I was going up the hill and the people behind me were looking at me weird at the end of the exit. I was probably shooting flames out the exhaust, and they probably thought I was doing it on purpose (lol).

- I did change my fuel filter about 4,000 miles ago.
- The air filter is good (K+N just cleaned).
-----------------------------------------------------------------
Hi,
I've recently read if you over oil the K&N air filter, it can deposit on the MAF sensor and cause driveability issues.
It can be cleaned but is very fragile.
Bob B

Hi,
I've recently read if you over oil the K&N air filter, it can deposit on the MAF sensor and cause driveability issues.
It can be cleaned but is very fragile.
Bob B

No MAF on the TBI Trucks (87 - 95).

//2000CAYukon

I believe that my fuel pressure stayed in the 13psi range. Does anybody think that it could be a knock sensor? I'm still looking for ideas.

I finally got around to checking the resistance numbers of my spark plug wires and found out the my #8 was the highest resistance and that was because the tip that plugs onto the sparkplug was very coroded. So I cleaned it up with dremel attachments and the resistance got lower. Now my truck seems to be better. But it still doesn't seem like full power. How can I test the coil?

The best simple test for a coil, is to have a buddy crank the engine while you have one or more plugs connected to the wires & laying on a metal engine surface. (Or hold the boot of the plugwire with well-insulated channel-lock pliers, so you can press the threaded part of the plug against a good ground, such as the bolt on a valve-cover or intake manifold). Just look at the quality of the spark. It should be fairly fat, and blue or white. Not yellow or orange.
That will give a good indication of adequate voltage & current. This test will not diagnose a coil that is going "open-circuit" when hot - they simply stop firing altogether, but that is not your issue.
Your knock-sensor is fine, and it was doing it's job by informing the onboard computer (ECM) of a knock, which resulted in the ECM retarding the timing to alleviate the knocking. there is some good advice above, the K&N filter oil can cause problems. I suggest you start with new AC Delco plugs, new plug wires, Distributor Cap & Rotor. Keep us posted, and good luck!

I had a similar prob where in overdrive & under a little load, it would
buck & sputter pretty good. A while back I got alot of parts to tune up
but had to send back plug wires. I thought maybe a tranny or lockup prob.
I replaced wires & prob gone. It seems the load was exposing a miss.

Sorry, I didn't intend to post that response to this thread