Engine running cold (per the gauge), rough at low RPM

[scottydl]

My Suburban has developed a really annoying "roughness" with the engine at low RPM / when cold, that I cannot pin down. It starts up and idles okay (no major surging), but I can feel some uneasiness and it'll sometimes stutter or die when I first shift into reverse to take off, or put an electrical load on like the window defroster.

Coupled with this is the fact that the engine temp gauge is always showing cold... 120-160 degrees even after I've been driving for 10-15 minutes, which obviously is not accurate. I have replaced every coolant sensor that I know to exist, with no change. Sometimes after 20-30 minutes of driving, the gauge climbs closer to the 210 middle point and things seem to smooth out with the engine.

It might be running rich, putting too much fuel through when it thinks the engine is still cold? But I've replaced those related sensors (I think) with no improvement yet.

I could type for 5 more paragraphs, but figured a video might be the most helpful... so I made and uploaded this:

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(if you watch and reply, feel free to do it here... you don't have to comment on the video itself)

I also have two "mystery connectors" that I found not attached to anything, that might be related? Those are shown in the video. To recap, here's what has already been done:

- Coolant temperature sensor (front of engine/intake)... replaced twice to rule out defective part
- Coolant fan switch (right engine block)
- Engine temperature gauge sensor (left engine block)
- Coolant flush
- Thermostat... replaced twice to rule out defective part
- EGR valve
- MAP / barometric pressure sensor

Any ideas are appreciated!

 

[94burbk1500]

I had an issue with my truck (5.7 not 7.4) running like **** from a cold start and stumbling until it warmed up a bit, cleaned my IAT sensor and it went away. It wouldn't explain the cold engine but it's easy to try and essentially free if you have carb cleaner or comparable solvent laying around.

 

[scottydl]

That sounds like a pretty simple thing to try... got a photo or description of the IAT sensor location?

 

[someotherguy]

There's no IAT on your 1995 model.

Replace thermostat with correct 195F and buy the best quality piece you can find. Drill a couple small (like 3/32") bleed holes in the flat portion of the thermostat to assist in bleeding the air out of the system. If you had an IR thermometer I'd suggest reading the actual coolant temp by scanning for max temp reading at the base of the thermostat housing/intake but if you don't have one, and are unsure of the thermostat condition, just replace it. They go bad.

Be sure the clutch fan isn't stuck engaged, running all the time. You would probably notice if it is, as you would hear the distinct roar of the fan when it should be relatively quiet.

Tried watching your video to see what loose connectors were but it would be more useful to those trying to help if you just posted clear pictures of them and their general location. Video keeps stuttering for whatever reason so I lost patience with trying to find where in there the loose connectors were.

Richard

 

[454cid]

My truck isn't a TBI so I'm not sure what those plugs may be for. It's not uncommon for vehicles to have un-used plugs, However. It's usually for options the truck isn't equipped with, as often the wiring harness is the same.

It looks like you've already covered some of the potential problem areas. It would be a good idea to check the temperature of the engine directly instead of using the gauge. The gauges are often not accurate. IR guns can be had cheaply. Do you get decent heat?

You might check the ground connection at the thermostat. It looks like the connector has been replaced there, but when? Is the wire in good shape?

You replaced the EGR valve... I've done read some thing that say some of the replacements don't work well. You might consider troubleshooting the EGR system.

Check the fuel pressure. Unfortunately, it's not as easy to check on the TBI engines. I believe you need to install some sort of adapter in the fuel line.

 

[94burbk1500]

Egr is easy enough to test, remove the vaccuum line from the egr, plug that line with a bolt/screw and see if it improves, you'll throw a code after driving like that for a bit, but it won't hurt anything.

 

[scottydl]

Replace thermostat with correct 195F and buy the best quality piece you can find. Drill a couple small (like 3/32") bleed holes in the flat portion of the thermostat to assist in bleeding the air out of the system.

The thermostat has been replaced recently... twice actually, because I thought the first replacement might be defective (stuck open and not allowing for proper engine warm-up). It seems pretty unlikely that 2 of them would be equally defective in the same way, but I suppose anything is possible? Before returning the first one, I also tested it in a pot of boiling water, and it appeared to open and close properly.

I don't have an IR thermometer, but sounds like that'd be a good addition to the toolbox. I just point it at the thermostat housing when the engine should be hot / thermostat open? I do get hot air through the interior vents, so that seems to indicate a properly heating/flowing system.

I'm familiar with the clutch cooling fan, from its operation while towing our trailer in the summer heat. That is definitely not running now.

My truck isn't a TBI so I'm not sure what those plugs may be for. It's not uncommon for vehicles to have un-used plugs, However. It's usually for options the truck isn't equipped with, as often the wiring harness is the same.

I was wondering if that was the case. Just wanted to make sure I wasn't missing something that should be plugged in on this 7.4L.

You might check the ground connection at the thermostat. It looks like the connector has been replaced there, but when? Is the wire in good shape?

If you're referring to the yellow hoop connector, yep I replaced that (and its wire) myself maybe a month ago when changing the thermostat. The old metal one bent and snapped off during the job. I'm not even sure what those grounds are for.

Egr is easy enough to test, remove the vaccuum line from the egr, plug that line with a bolt/screw and see if it improves, you'll throw a code after driving like that for a bit, but it won't hurt anything.

I'll give that a try. EGR valve has not been replaced twice (by shops, not me) since I've owned this vehicle. That's about 3 years, but only about 15k miles driven in that time. Should these things be going bad so often? They are EXPENSIVE.

FWIW both times when the EGR was bad before, it threw a code and SES light came on regularly. That's not currently the case. Maybe the SES bulb burned out (ha).

Thanks for all the suggestions, keep em coming!

 

[scottydl]

With the crappy weather in the Midwest and my lack of time to troubleshoot these problems, I took it to a small local shop about a month ago. They still haven't figured out the rough running and general bogged down acceleration. Tried different plugs, checked many of the items I checked, replaced throttle body gasket (thinking air was getting sucked in there), replacing thermostat AGAIN thinking maybe seal is faulty and causing coolant to flow constantly and engine to run cold.

A buddy of this mechanic is apparently (from what I've told) a very experienced GM tech who works at a nearby GM/Cadillac dealer. He stopped by with some portable diagnostics and hasn't been able to spot-solve it either, but seems fairly confident he'd be able to figure it out if given an exclusive look. Of course that'll be at the dealership ($$$) but I'm running out of options.

One possibility mentioned is an obstruction of some kind in the exhaust system. Anyone ever heard of that? I asked about the catalytic converter, and mechanic said it's hard to tell because the cat and muffler is all one unit and he can't really get a good look. At their smaller shop, he is reluctant to pull the exhaust/headers down because of the rust and risk of breaking parts/bolts that won't easily go back together again.

 

[east302]

I’m not sure if the OBD-1 can report it, but I’d use a scanner to read the coolant temperature sensor output. If it’s normal, then I’d suspect a bum instrument cluster or just a normally inaccurate gauge. For what it’s worth, that’s where my two 98 models read on the temp gauge.

You can test exhaust pressure upstream of the catalytic converter by removing the oxygen sensor and installing a pressure gauge. Here’s the spec for a vortec, not sure if the TBI is the same.

What is your voltage as measured at the alternator? Go with that over the dash gauge.

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[scottydl]

Repair update: it was the fuel pump. That's it! After all the speculation from different mechanics about head gasket, blocked exhaust or catalytic converter, ECM malfunction... the last place I took it (a Chevy dealership) correctly diagnosed as the fuel pump and it's running fine again. Just about cost me a mortgage payment, but that's what I get at a dealer. I obviously couldn't figure it out myself, had already taken to 2 other smaller shops, and was out of other options.

One of the smaller shops was able to determine that the thermostat was missing its outer seal/gasket, so that was replaced again and I believe has fixed the temperature problem too. The gauge still looks like its running colder than normal (well under the 210 center point), but it's been ungodly freezing in the Midwest recently. Next time I find a good price on an IR temp gauge (probably Harbor Freight) I'll pick one up and test it.