94 4.3L low power, poor gas mileage

thx1138v2 · November 26, 2011

I bought this truck new from the dealer and have done most of the maintenance myself. It now has 202K miles. I recently had my original factory transmission rebuilt by the best shop in Houston. They found that the motor mounts needed replacing but didn't do the work so I replaced them. At the same time I did a kind of light going over the engine compartment checking wires, hoses, etc. and didn't find anything out of order.

I also recently had a brake error code that I solved by replacing the switch under the dash by the brake pedal.

I don't have any error codes now but took it on a road trip (260 miles one way) last week and got poor gas mileage - 12 MPG as opposed to my normal 17MPG and it feels kind of sluggish. I replaced the plugs, spark plug wires, rotor, distributor cap, and coolant temperature sensor (by the thermostat) before the trip because it was time for that anyway.

It runs fine when I first start it but once I get it up to operating temperature (195*) it feels sluggish when starting from a stop and at aroung 2200 RPM I feel a kind of surging back and forth almost like you feel when your brake rotors need turning and you press the bakes. But this is under accereration. I don't, however, see it on the tachometer but it's not a heavy surge - more just a seat of the pants feeling.

It has throttle body injection. The tailpipe is not blackened with soot. I ran some injector cleaner gasoline additive through it before the road trip. The fuel filter was replaced about 6 months ago.

All the gauges read nominal. I've been out of work for a while so I don't want to just start throwing new parts at it.

Any help identifying the problem will be greatly appreciated.

jdaniel83 · November 26, 2011

Have you had the catalytic converter checked? Sounds like it could be clogged or partially clogged causing the poor performance/MPGs. And I could be wrong with the pre-Vortecs but you should probably be getting better than 17 MPG out of a 4.3L. My '97 4.3L has over 260K on it and I get at least 17-18 city and around 20-22 highway out of it.

GMstrong · November 26, 2011

What year?i would check fuel pressure,map sensor,TPS sensor,IAC valve,Depending on whether its the tbi or vortec go to the jyard and get a set of injectors IF its TBI.or just run GM upper cylinder cleaner threw it.<-- that stuff is 10x better then sea foam tested both on my truck before i sold it which had 270k and sea foam smoked a little with no notice in performance or driveabilty but a huge difference with the GM cleaner.

92rado · November 26, 2011

Also check your coolant temp sensor. Nevermind just read that you changed that.

thx1138v2 · November 27, 2011

@jdaniel83 - I just had it inspected in September and it passed the emissions test easily. I'm assuming a clogged catalytic converter would affect this and maybe give an O2 sensor code which I don't have. Is that correct thinking?

@GMstrong - It is a'94 with TBI. I found a good youtube video to show me how to check for dripping injectors. I'll check that out today. I don't think that is the problem because I don't have any soot on the tailpipe but it's worth a check.

Do I have to get the GM upper cylinder cleaner from a GM dealer?

I try to stay away for junk yard parts because I don't want to buy someone else's problem. I usually try to buy new but if not available will buy rebuilt. I'll look at the other items today. Do you know of a good web site with diagnostic procedures for those items?

@92rado - You got me thinking about this and maybe I got a manufacturer's defect with the new one because it runs OK when cold. On the other hand, the temperature gauge seems to be working correctly. I haven't traced out the scematic but I'm guessing that the temperature gauge responds to the CTS. I'm now wondering if I need to do a computer reset because of the new CTS. I guess I can try it.

Thanks guys. I'll post back.

jdaniel83 · November 27, 2011

I had a similar problem that you're describing with my '97 about a year ago and even though it passed the emissions test easily it ended up that some of the "honeycomb" material inside the catalytic converter had come lose causing it to be partially clogged. There were no codes so I couldn't figure out what was wrong till I had a friend sit in the truck and rev the motor while I stood by the tailpipe and felt the exhaust pressure. I could tell that it was pretty weak so I took it to an exhaust shop and he figured out the cat problem and replaced it. Got the power/MPGs back immediately. Not sure if that's what is wrong with your truck but it gives you something to check.

wavery · November 27, 2011

Have you had the catalytic converter checked? Sounds like it could be clogged or partially clogged causing the poor performance/MPGs. And I could be wrong with the pre-Vortecs but you should probably be getting better than 17 MPG out of a 4.3L. My '97 4.3L has over 260K on it and I get at least 17-18 city and around 20-22 highway out of it.

Ditto on this one.........@200K miles.....just replace the Cat conv. If it isn't clogged, it soon will be.

thx1138v2 · November 29, 2011

Yep, looks like the catalytic converter. The injectors checked out OK with a timing light - no drips and a nice cone shaped spray. CTS was good by checking the resistance - 1900 Ohms at ~120*. The MAP and TPS sensors checked out OK. I didn't get around to the IAC. BTW, does anyone know what range of vacuum readings off of the manifold I should get?

The clincher to ID the catalytic converter was low pressure at the exhaust pipe with the engine running.

So I need a new catalytic converter. I can get a universal one for ~$80 but I don't see any way to fit it on the pipes because the front is right where the 2 pipes from the manifold come together. The all-in-one unit is ~$270.

Does anyone know of a kit I could buy so I can use a universal converter next time, e.g. manifold pipes, universal converter, and tail piece (3 pieces) plus clamps or flanges, etc? So next time I would be able to just replace the converter?

Or maybe use the pipes I have and weld on some flanges and use a bolt up converter? Any ideas?

Many, many thanks.

92rado · November 29, 2011

I just bought a magnaflow high flow universal and had it put in for $40.

wavery · November 29, 2011

I've heard of some dirty, no good, low down people by-passing it....

thx1138v2 · November 29, 2011

I haven't yet actually removed it. Waiting on the PB Penetrating Catalyst to do its magic on the manifold nuts. After thinking about it, I guess the blockage could be in the muffler. I'll find out when I get it apart.

@wavery: Were those dirty, no good, low down people happy though? And more important, will it pass inspection next year? I don't want to have to do this twice. I'd much rather spend my time with my 2 year old grandson.

@92rado: It's a thought. I have a stick welder but I'm not very good with it. I'm concerned about burning through the pipes. Anybody have clues about what amperage and rods to use?

92rado · November 29, 2011

It is extremely hard to weld exhaust with a stick welder. I would just pay to have it done so that way if you have any leaks they will fix it. But it's always cheaper to buy your own muffler or cat , rather than buy it from the exhaust shop.

Also when my cat went bad it started to smell like sulfer when it got hot.

wavery · November 30, 2011

Can't you find a clamp on cat?

thx1138v2 · November 30, 2011

@wavery: That's what I would really like to do but there is only 1/2" between the O2 sesnor fitting and the front weld on the converter. I guess I could cut it a couple of inchs back from there and try to grind the weld off hoping there is more pipe inside the converter.

Wrestled with a broken stud most of the day but finally got it out without taking the manifold off. There's a bracket between the alternator, power steering pump, and the manifold that had to come off before I could pull the manifold which, because of the way things are mounted, would have required me to pull more belt driven accessories off before I could get the bracket off. I did, however, manage to get the broken stud out.

Not sure where I'm going with the catalytic converter. Tomorrow....