bjafeman

Not quite sure what's going on with my 91 I6, but the operating temperature never really gets into the normal range. It always seems to hover just below it unless I've been driving it hard, with the AC on, and up hills. Even then, it just barely dips into the low end.

The mileage is bad (about 10-12 mpg), it smells kind of rich, and I think it's pretty short on power. It feels like it's about to start missing on a steep incline/heavy load. Water pump was recently replaced, not losing any coolant. Tested the ECT sensor, working fine.

I think I may have a bad EGR or EVP as well. I have a vacuum tool on order to test the EGR later. When I checked the resistance between the EVP and the VREF it was 3,500. According to the Haynes manual I have, it should have been @ 5,000.

Last but not least, the EGR tube that runs from the engine bay to the cat has rusted away and is pulling air from under the truck. The previous owner kindly glued a bolt into the hole that was left in the cat.

I know I need to reconnect the EGR with the cat. Was hoping I could do that with a little tubing and a welder (basically just patch the two ends). Would like to test things out so that if the EGR does need to be replaced I'll just get the whole tube with sensor combo and not worry about creative metal work.

Advice on next steps?

Just call me Sean

iTrader: (1)

Your thermostat may be stuck open. Open the radiator cap (when cold), start the engine and watch the coolant. If the coolant circulates right away it's stuck open. This will cause the truck to run rich as the computer thinks the truck hasn't warmed up. You should get a code if the EGR or EVP is bad. You can unplug the EVP and see if it stores a code. The EGR doesn't connect to the catalytic converter, it connects to the exhaust manifold. The pipe to the cat goes to the smog pump, and it doesn't matter if it's disconnected except for smog inspection. It doesn't draw in any air, it pumps it out from the smog pump. There will also be a leak at the cat from the hole, which would be annoying but not a problem. If you really want to fix it and don't want to have to weld it you can get a compression fitting of the proper size from Home Depot if one is available. I'm not sure of the diameter of the pipe but if it's 1/2 or 3/4 then use a piece of copper pipe also to reconnect the pipes.

Good old Bill

Agree with Sean, its mandatory for the engine to be at operationg temperature to be in closed loop. Cold engine=open loop or rich mixture.

bjafeman

Yeah, the thermostat was my first guess. Forgot to mention that I replaced it a few weeks ago. While I haven't sat and watched the radiator to see when the stat opens, I know that the top hose only gets hot to the touch after trips of more than 20 minutes or so. I'm reasonably certain it's functioning properly.

Wondering if I tested the ECT properly. All I did was watch the resistance drop with a temperature rise. The manual I have doesn't give specific numbers to look for, it just says "look for change".

Fast Hampster

Well it looks alright generally, but.. I think that 3-4 minutes of driving is enough for any engine to warm up, it means about 190°F temp for whole cooling system. I'd wanted to put aside the temperature theory and start thinking of something else, but i'm low on ideas. The only other possibility i can think of is MAF sensor - that would make an engine run reach too. I guess that's it's worth it to take a look at it, but mostly i think that something's wrong with temp sensor or temp generally.

Just call me Sean

iTrader: (1)

Just because the thermostat is new doesn't mean it isn't bad. I learned this the hard way.

bjafeman

Yeah, the more I think about this the more I think it's got to be the thermostat. It's kind of a pain in the butt to remove the thermostat and I would like to avoid doing that unless necessary.

How does this sound as a thermostat test:
1) -with a cold engine- Disconnect top tube from radiator and hang it into a bucket or some sort of collector with a thermometer in it.

2) Start engine and wait for coolant to go into bucket.

3) If coolant goes into bucket immediately, or at a temp below 185 or so, then I probably have a bad thermostat.

Just call me Sean

iTrader: (1)

When cold the water in the radiator should remain relatively still with the exception of the water that bypasses through the heater. Remove the radiator cap and watch for coolant circulation. If it's pumping quickly while cold the thermostat is stuck open. When the water level is 2 inches or so from the top of the radiator it's easier to see the water pumping because you will see it streaming from the side.

I've never tried it your way. It might work, but many thermostats have a bypass hole in it to allow for coolant circulation in the event the thermostat gets stuck closed. If you can remember if the one you put in had that little bearing looking thing rattling around then that's the bypass.

bjafeman

I decided to opt for the test where you just look down the fill tube. Much easier. Like you said, the water was relatively still.

Tried burping the radiator too (front end on ramps, engine at temp, heater on hottest setting). There was some trapped gas that came out, but no difference for temp gauge.

Starting to think I have a bad coolant sensor or bad connection to it. After running the hill a few times with full heat and full AC on the way home, the radiator top tube was way too hot to hold on to for more than a second or two and was also pressurized. Voltage from PCM to sensor is within specs, but connectors looked a little crudded up. Going to go clean those off and check my MAP sensor.

Will post update.