Bizarre drivability issues 1995 K2500 454

Intragration · May 27, 2017

Hey, I'm new here, but a confirmed GMT400 (425) guy. Truck is a 1995 GMC Suburban 2500 4x4 with the 454. I bought it several years ago with about 180k miles, and today it has about 230k. It's from California and is pristine, and the previous owner replaced the transmission at some point, "proactively" he said, but I'm guessing it had something to do with this issue, which seems to be ongoing.

It starts and runs fine, and generally drives ok. If I give it a moderate amount of gas, say to catch up with or pass someone, or if I give it enough gas to crest a pretty steep grade, the problem starts. Most evident is, it will start shifting incredibly hard in mild driving, like it has a stage three shift kit. If I get on it, it acts like it's down on power, or like the transmission is slipping, maybe 40-50%. When I come to a stop, it has a mild, non-engine shudder, almost like there's something binding (tight and expecting an irrational physical condition?) in the transmission. This will continue until I turn it off for about 30 seconds. If I do this, it goes 100% back to normal. If I get on it again, this all starts again. It all starts from this moderate load condition, results in exactly the same various symptoms, and reliably goes away if I turn it off for 30 seconds. I can drive it for an hour with no problems, get on it, and the problem starts, or I can start it cold, warm it up for three minutes, get on it, and the problem starts. It's specific to the driving condition and nothing else, and it reliably does it and stops doing it upon 30 second shutdown.

I'm mechanically inclined, I do engine swaps on my fun cars, which are all old and carbureted. On this truck, I usually take it to my mechanic, for ease and quick turnaround. I had replaced the TPS at one point, and that seemed to help, but then the problem came back with a vengeance. Left it at my mechanic for a week after that, and he also found it was the TPS, he witnessed crazy varying signals, and so he replaced it, and it was fine again for a while. Now the problem is back, and I'm guessing it's the TPS again.

Has anyone else experienced this, with TPS sensors that just keep going out? Is there maybe something that's causing them to fail? I'll get to the bottom of this, or I'll put in a crate 502 and a trans controller and be done with it. Just hoping to get by for a little longer. At 230k miles, it's really just getting broken in, zero other problems with it. It's only 240 HP and an 8,000 pound truck, but it should be able to get out of its own way without all this drama. Any help is greatly appreciated.

grampadirt · May 27, 2017

If you do replace the TPS make sure it's a Delphi or ACDelco.There are a couple of other things to check out....the MAP sensor being one and the other is the ICM(ignition control module)and you'll find it under the rotor in the distributor.Always buy Delphi or ACDelco parts,don't use BWD or any other aftermarket parts.

BTW I have the same vehicle as you and love it.There were times when I wanted to drive off a cliff but stuck it out and solved all it's running issues.Now it runs great and has for a few years and use it for towing my toy hauler and doing some moderate off-roading.

barry G · May 27, 2017

big motor on the little throttle.

Maybe rummage ebay for the oversized and a spacer. it may not be power gains, but proper.

the 305 is the only one I know of that is full speed for decades on the same injection.

if all gadgets are working, like a/c, power steering cooler, oil cooler.. the trans may be a bit too warm.

I even had to modify the 305 version of pwer steering cooler after a/c was up and running.

A lot of warmth.

in BTU value, the 454 is just absolutely huge.

keep cold air feeding it.

Intragration · May 28, 2017

Thanks for the responses guys. Gramps, I had seen your truck around here, yep, these are great vehicles. I like the simplicity of them, although it seems the '95s are kind of an oddball year.

I will be doing some testing to see if the TPS is bad, and will also check out the MAP, I had heard about that possibility as well.

I'm going to have to track down the Delphi or Delco parts, for now I have a cheap TPS replacement, because it has to be better than a failed one. My main question is, what is a speed governor, and how do I know if I have one? The LPS TPS that I found offers a "with speed governor" and "without speed governor" version. The parts clerks have no idea what this is, no idea how to tell the difference, and frankly, neither do I. Thanks again for any and all help!

grampadirt · May 30, 2017

Looking at Rock Auto I see the ''with'' or ''without'' TPS made by Airtex and have no idea what the operating difference is.I'd just get the ACDelco part and be good with that.I've never heard of these either.

I use a program called Tuner Pro RT to monitor my engine,it's a little involved setting it up but once that's done it can help diagnose many problems.In fact it helped find a TPS problem I had once.Maybe three years ago(or so)I was getting a hesitation/jump when stepping on the gas pedal and was driving me crazy so after firing up the laptop,plugging in the ALDL cable and opening that program I watched the TPS gauge in that program closely while a neighbor wiggled all the wires in the area of the TPS and saw a jump in the numbers.Asked him to do it again slower and found the slight break in the wires going to the TPS.Well apparently I had re-routed those wires to under the EGR valve and because that thing gets quite hot it had melted two of the three wires.So I replaced the TPS,soldered the connections and I was good to go and of course I re-routed those wires lol.

So if you are interested in this program go to http://www.tunerpro.net/ and have a look around.

The '95s were a bastard year and have many aspects that are this year only.Great for us huh? lol.

One more thing...I have a couple of videos of this program in action if you'd like to see one.

Intragration · May 31, 2017

Thanks for the additional info, grampa. So here's where I'm at today. I did replace the TPS with the crap Autozone unit, I could get my hands on it quickly, and it had to be better than the failed one. The hard shifting after heavy throttle at 20 MPH got 90% better, but the flaring and lack of power were still present. I did a little road testing yesterday, and the CEL (MIL, SES, whatever) came back on, first time in a while. It indicated 36 (Idle speed control actuator malfunction) and 85, PROM error. Now I figure I'm getting somewhere.

Pulled the computer today, re-seated the prom, and took extra care with reconnecting. (there was a bit of mysterious gunk on one of the connectors, don't think this was specifically related) After this, for all realistic intents, it's 100% better. There is still a weird transmission thing in highly unusual, race-type circumstances, but this is a Suburban 2500, racing isn't my goal. Driving around town, there is no flare, power is right where it's supposed to be, it's crisp and responsive. What's most strange is that, although the CEL is not lit now, there is STILL an 85 PROM error reported. Maybe there really is a problem with the PROM itself. I'm going to keep an eye on this, and plan on replacing the TPS with a Delco/Delphi, and possibly the PROM, although at this point, that's going to be either a tuner's stock-ish chip, a Hypertech, or something equally stock-like.

Any additional feedback is greatly appreciated.

grampadirt · June 5, 2017

Go to http://www.alldatadiy.com/ and join up,there's a lot of info there on our trucks.I looked at Code 85 and it references the transmission,maybe it will help your trans problem.