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My 1987 Chevy Truck Won't Go, What Is Wrong???

D

Dave1

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Winter Springs, (Central) Florida
My 1987 Chevy 3/4 ton Suburban 454 with TBI, 100K miles, etc. will crank and run OK at low speed but will not go over about 60mph. When loaded pulling a trailer it won't go but about 45mph. If you mash the gas all the way to the floor it will start slowing down. What is wrong with it?

This truck has a single exhaust with what I believe is a catylitic converter. There is two muffler looking things in the exhaust pipe and one smaller round thing near the tail pipe. I have heard these things can clog up and hurt performance. I have owned this truck since about 40K miles with no problems. The exhaust has never been off or replaced. Could this be my problem? or fuel filter clogged?? or plugs??? or what???

Thanks for any feedback.

Dave
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did you check your distributor? your advance might be messed up, it probably has an electric advance on it, check the contacts on the distributor's control module (i forget the real name for that box), its on the dist itself. There's a wire by the firewall that you can use to disconnect/reconnect the advance as well, might be a problem there.

just an idea for ya
 
I believe one non-scientific test is to try and run the engine at higher RPM's in park.

The real test is to pull the O2 sensor and hook up a pressure gauge, and see if the pressure climbs a large amount as you depress the accelerator.

I don't have a newer truck manual, I believe the car manual from '88 states anything over 3-4psi is an indicator of an exhaust problem.
 
could be fuel filter. one of the most over looked items. i did a master tune up on my old truck except fuel filter ran o.k. changed filter and would clean tires off. but like all other check over the tune up stuff.
 
I didn't check the dist, it used to run very well so didn't think the dist would be a problem. The ECM either works or it doesn't, usually no in between that would cause a slow run condition. It runs OK now just will not run over about 60 and starts to bog down if I step further down on the gas.

I remember a while back somebody mentioned they had a clogged cat converter and it had similar symptoms as mine does now. That is why I thought that may be my trouble. This truck has the original Cat converter on it. Is it common for the converters to clog up around 100K miles? If we suspect that is the problem I guess I have little choice but to replace it. If I remove it, I think the engine light will come on and stay on. Don't want that.

I haven't changed the in line fuel filter in a couple years but think a clogged filter usually will allow a build up of fuel pressure and engine speed up after backing off the accelerator for a few seconds. That doesn't happen now.

The check engine light has come on intermittently for years. Had it checked one time and it was a code for a throttle sensor of some kind and I have ignored it. Maybe it could be related but doubt it.

??????

Dave
 
Wouldn't hurt to check the TPS voltage, make sure the sensor is operating correctly.

Never seen/heard of one, nor looked, but I would imagine there is a pressure tester adapter fitting that threads into the O2 bung. No reason to mess with the exhaust if it's not the problem, backpressure test will tell.
 
one tell tell sign of a bad cat is your exhaust smelling like rotten eggs...this is not always the case, but does it smell funny? also crawl under the truck and smack the cat..hear anything rattle around inside of it?
 
I very much agree to not do any unnecessary work. That is why I am asking questions here trying to narrow down the possibilities for my problem before doing anything. I am trying to educate myself before starting any work.

Not being a mechanic, I do not know hwere the O2 sensor is nor do I have a pressure tester to run the back pressure test with. I do understand the concept and reasoning, just don't have the proper equipment to test as you suggested.

You guys on this forum are a great resource for information. Sometimes I cannot keep up with you though. Lack of knowledge, experience, and adequate tools and test gear hinders my efforts so I have attack each situation with very basic approach and tools. Sometimes when I pry into each situation, asking alot of questions, sometimes the problem and solution becomes more evident. I haven't got there yet with this problem but still thinking.

To my way of sorting this thing out I think the problem more than likely has to be lack of adequate fuel supply, malfunctioning ignition system, or clogged up exhaust system. In my inexperienced opinion the ignition is the least likely culprit because it cranks and runs fine at lower speeds. I did replace the fuel filter, pump, tank, etc. a few years ago so I would put that as a #2 probability. That leaves the exhaust system as the possible culprit. In that system the cat converter has been known to clog with carbon, rust, etc. and cause these problems. I think that will be the first place I start with. I'll let you know what I find.

Thanks for everyone's feedback.

Dave
 
Since I don't really deal with shops, I'm not sure how well equipped certain ones will be. A clogged converter test should be pretty commonplace IMO, so you might try calling some shops to see what they will charge to do the things you ask, and if they can do them in the first place.

"Lacks power" is probably listed in the service manual for the truck, which would be something to look at as well.
 
Here is a real easy test for a plugged cat. Just drill a hole before the cat, and one more behind it. Start up the truck and put your hand over each of the holes. If you feel a big difference in pressure then the cat is clogged. :eek1:
I took my truck in for similar problems, and the first shop put a pressure gauge in the O2 sensor hole and diagnosed a plugged exhaust, but they don't do exhaust work. So I took it to the muffler shop and I watched them drill the holes to check it. My truck had two cats, one before , and a little round one after the muffler and both were clogged. Just use a small [ 1/4 "] drill bit incase the cat is good, then all you have to do is weld on a nut, and screw in a bolt to seal it back up. ;)
 
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