1991 GMC runs poorly after engine is warm
 

Hello there!

I bought a '91 GMC 1500 automatic with a 4.3 about 1 year ago. Everything seemed fine until about a week ago.

At first, it started to stumble/hesitate every so often. As time wore on it got worse until it flat wouldn't run when I put it into gear without dying. Sometimes it would start ..... sometimes not. I replaced the fuel filter without any improvement. Friend told me to check the Idle Air Control which I ended up replacing. After replacing the IAC it started, idled and ran like a champ (actually better than it ever did before) ........ until the engine obtained normal operating speed. After it warms up it stumbles/hesitates any time you apply pressure to the accelarator. sometime it seems to cough (not really backfire) through the carb, and the engine light sporadically glows. When checking the codes it gives an O2 fault.

OK, now comes the tricky part. I always suspected that the engine had been replaced at one point in time but really don't know that for a fact. Some of the emission systems were removed like the catalytic converter and the air pump. After attempting to check the Oxygen sensor I found it didn't have one. I'm assuming it should have been found underneath just downstream from the exhaust manifold on the passanger side. I found a plug in its place. I'm starting to wonder what else could have been removed if anything.

Should also mention that I also tried to check the EGR valve by letting it idle and lightly pushing the diaphram out until the engine started to die.

Since I'm a greenhorn at being a mechanic I'm not sure where to go from here since the truck seemed to run fine until recently. Hopefully someone can point me in the right direction.

THANKS for any help that can be offered!

Re: 1991 GMC runs poorly after engine is warm
 

I see it’s your first post, so welcome to the message board.

Let’s address the check engine light first.

Being you said the truck run ok for a year, until a week ago, my guess is there is an O2 sensor on it, but you just have not found it. I’m assuming the check engine light just started going on as the problems started.

Did the check engine light just start coming on as the problem started in the last week?

Re: 1991 GMC runs poorly after engine is warm
 

Thanks for the welcome chevytech ......

Yes the light never made a appearance until the problems started. I've looked in several places for the o2 sensor without finding it. Is it possible it doesn't have one? Where else might have they have installed this rascal? Also, is it possible that even though the code says O2, that something else could be the main culprit ......

Like I said in the earlier post, I'm not exactly a pro at this mechanic thing but have puttered around with several other different vehicles. This is the first PU I've ever owned, but it just seemed a little odd that when I 1st popped the hood that I could actually see the engine without a lot of the usual clutter and controls you see under the hood now-a-days. Reminded me a little of the cars back from the mid to late 70's.

Re: 1991 GMC runs poorly after engine is warm
 

The O2 sensor is in your exhaust pipe or exhaust manifold. You may also want to look at your temperature sending unit and/or thermostat. If it is sending a wrong temp to the computer the computer may be wanting to compensate for something that does not need compensated for. Same as when a knock sensor kicks it it will tell the computer to change the timing.
With your truck being a '91 it will have an O2 sensor unless someone has removed it to save a couple of bucks. But without it the computer will still act as there is one and compensating for it being bad. Look at your driver side exhaust manifold and if not there it will be in your exhaust pipe before the catalytic converter.

Kevin

Re: 1991 GMC runs poorly after engine is warm
 

On my 93 4.3 the O2 sensor is just before the cat on the passenger side. It was angled up toward the cab at about a 30* angle. I would assume yours should be in the same place if the PO didn't remove it. There should be a wire for it somewhere around that area too.

Mike

Also Welcome to the board!!

Re: 1991 GMC runs poorly after engine is warm
 

Being it ran all that time with no check engine light, I think it’s a pretty sure bet there is an O2 senor on your truck.

All the information I checked shows the O2 sensor in the exhaust pipe in front of the catalytic converter for a 4.3 engine.

Here are some links you can check out.
Component location
http://www.autozone.com/servlet/UiBr...3d800870d8.jsp

O2 sensor information
http://www.autozone.com/servlet/UiBr...3d80086e71.jsp

Re: 1991 GMC runs poorly after engine is warm
 

I'm with Kevin. The coolant temp sensor failure will cause this very symtom.

Re: 1991 GMC runs poorly after engine is warm
 

Just like said before, on the fuel injected enigines there is no choke so the coolent temp sensor is the choke tells the computer to run rich and if bad will run ok at cold but really crappy after heating up Do you smell that rotten egg smell???

Re: 1991 GMC runs poorly after engine is warm
 

Thanks for the replies everyone. I remembered that I didn't clear the codes after replacing the IAC ..... so I did that and have drove the PU a couple of times and both times it went about 3 miles before showing symptoms again....... this time without the engine light coming on.

I have to admit that I never would have thought of the temp sensor. As far as a smell goes, I did notice a strange smell after replacing the IAC but it didn't really smell quite like rotten eggs but it did have a sort of acrid odor.

As far as the O2 sensor goes, I can safely say that it is not in the exhaust before the converter (exhaust had been totally replaced and converter was left out ...... as well as the air pump). Also checked around the exhaust manifold on both sides and failed to locate it at either location although I haven't been able to fully access the passenger side fully yet.

Hopefully I'll have more time to check things out a little more tomorrow.

I truely appreciate everyones input!

Re: 1991 GMC runs poorly after engine is warm
 

Good to go hope it all works out for you. let us know what you find out....:metal:

Re: 1991 GMC runs poorly after engine is warm
 

Ok here's an update. PU is back to running basically the same it was before the problems began. Yahooooo! LoL! But ........... the engine light comes on with the O2 sensor code after approaching around 45 MpH.

When I went to check the coolant temp sensor I found that the connector was a little corroded. I cleaned the terminals and ohmed out the sensor and checked the voltage at the connector and everything seemed to be fine. Evidently the corroded connection was the culprit for the bad conditions after the engine warmed up.

Back to Oxygen sensor .......... Where the heck is it? LoL! I've checked where it should be but can't find anything in the exhaust system (pipes, manifold) except for the plug that was installed when the new exhaust was put in.

I did find a couple of other sensors located on the drivers side. One is toward the bottom near the oil filter (Oil sending unit?) and has what appears to be a black wire. The nut on it basically looks like a ball of rust so I would hate to touch this. The other sensor is located between the #3 and #5 spark plug and has a single wire that appears to be a light to medium blueish wire. Could this possibly be it? The wiring diagrahm that I have says the 02 sensor has a purple wire though.

If some one removed the 02 sensor from the system would their be a reason for this and wouldn't this cause all sorts of other problems. If I did have an Oxygen sensor go AWOL how hard would it be to install one without messing something else up? Does seem strange sense I never had the engine light come on before the other problems started.

Re: 1991 GMC runs poorly after engine is warm
 

Glad you are making progress.

The knock sensor should be located in that area on a 4.3 – yes leave it alone. I think the knock sensor wire is dark blue.

The one between the #3 and #5 spark plugs is the temp sending unit for the dash gauge/light.

Yes, purple.

Be looking for the O2 sensor in an exhaust part where the exhaust gasses will hit it. It does not screw into the engine block. O2 sensors on some engines are in the exhaust manifolds but yours should be in the exhaust pipe. It takes heat to make the O2 sensor work, so it they are close to the engine.

Is it possible that what looks like a plug is the old O2 sensor broken off?

This is why I thought I wanted to get to the resolution of this issue. The system is dependent on a working O2 sensor to go into closed loop operation and work properly.

Re: 1991 GMC runs poorly after engine is warm
 

I'm 99% positive that it is indeed a plug as the top is smooth and has every appearance as that of a threaded plug. It would at least put my mind at ease if it were broken off but I do not see any signs of wiring anywhere near this location. I have a sinking feeling that it was removed for whatever reason by a previous owner. I guess I should go to the module and see if I can find the wiring there. Looks like I may be putting a sensor in. Which brings me to another question. Does that mean that an air pump should also be reinstalled?

Re: 1991 GMC runs poorly after engine is warm
 

4.3 liter engines had trouble keeping the O2 sensor warm enough to function properly under some conditions. Some of the first heated (3 wire) O2 sensors were used on 4.3 engines. Under SOME conditions the AIR pump does help warm the O2 sensor. Air pumped into a rich exhaust stream causes combustion in the exhaust system making it hotter.

All that being said if it were my truck, and someone had removed the O2 sensor, I would put an O2 sensor in it without putting an AIR pump on it, and see if I could get by with it. Before I spent money on an AIR pump, I would be adding a relay so I could use a heated O2 sensor, if 1991 4.3 liter engines don’t use a 3 wire sensor.

Re: 1991 GMC runs poorly after engine is warm
 

You do not need an air pump. Most air pumps were on trucks on the west coast. If you see a plug in the exhaust pipe, this is where the O2 sensor should be. The thing is that you will need to find the wire that goes to it. I can take a picture of my '89 or '93 if you like to show which direction it is coming from.

Kevin

Re: 1991 GMC runs poorly after engine is warm
 

I can take pics of my 93 too if needed. I have to do an oil change friday anyway.

Mike

Re: 1991 GMC runs poorly after engine is warm
 

Okay..... finally had to post an update. YEEHAW! Finally found the wiring I was looking for! Whoever removed the O2 sensor had tucked the harness end up above the transmission out of sight. Cleaned the sucker up, added the 02 sensor and with my other repairs ....... everything is fine and dandy now!

Well now I have a brake problem. Boy when it rains it pours! LoL. Back drivers wheel started locking up so now I have to figure this out. Guess my first process will be replacing the shoes and the cylinder.

Anyway, I wanted to make sure I thanked EVERYONE for their input! I have to admit that his is the best place on the "net" to get some help! THANKS again!

Re: 1991 GMC runs poorly after engine is warm
 

Thanks for the update.
I bet it runs better now then it has since you’ve owned it.

Re: 1991 GMC runs poorly after engine is warm
 

I haven't been on for a few days but glad to hear you got it fixed. With everything running right you should notice an increase in your gas mileage also.:metal: