Another 1991 Suburban

[aardvarcus]

I am no expert with the 8.1L paired with the 4l80E or on the standalone ECM you are using (on my 8.1L/Allison 1000 2500HD there are separate ECM and TCM chips). I am assuming your ECM is running the transmission too. The engine doesn’t need to know it is in 4wd, but the transmission does. Thus you do need to connect your transfer case range signal to the ECM (which is acting as the TCM) properly because of the auto. If there isn’t already a stock pin to steal the signal from, easiest way would be to use a 5 pin single pole double throw relay. Use the 4wd low engaged signal on your transfer case to run the input signal on the relay. When it is “on”, have the relay connect the output to ground. When it is “off” have the relay connect it to 12v or whatever voltage signal it wants. That should straighten up your issue.

 

[sean257]

Thanks again for all of your help. I have considered the suburban done now, with the exception of the AC hoses, which do not really matter since it is in the 20's right now.

To fix my pcm/4low issue I did a simple toggle switch next to the floor xfer case shifter.

I ended up changing out the radiator to a new aluminum unit with electric fans, and added a transmission cooler.

I am running the trans fluid through the radiator, then to the cooler, then return to trans. Seems to be working, but Ill find out for sure come summer. Right now, when the air temp was ~60 deg, my trans got up to 212 deg running backroads out of overdrive. On the highway, it stayed around 190.

Engine compartment, as I call it complete:

AC hoses are in that pic, but they had a huge leak, so back at parts store to fix or replace.

At least the front fits in the garage to keep me out of the rain.

 

[sean257]

Ended up going with a single exhaust exiting on the rear driver side. Quite a bit quieter than the straight pipes with cherry bombs, but I like it. Muffler shop put a converter on in case I was undercover EPA. The brown crap on my new pipes is transmission fluid. (When I was trying to figure out my shifting problems I took out the back cap on the transmission, where a speed sensor would go if it was a 2wd, out of curiosity. Then I just hammered the cap back in That doesn't work...)





Clip with sound:


Back in use:

 

[MTCK]

I've been following your build and the suburban looks great. How do you like the power train combo? Any idea what your rig weighs now?

 

[Larry]

Looks great Sean! Hope your happy with the 8.1L.

 

[sean257]

Thanks! I like it a lot. I daily drove it for a month or so to work out the rest of the gremlins, and wish I could just keep it at home. I have not weighed it since the swap, but imagine it is around 7k lbs.

 

[dblsprt68]

David - the guy that built my ladder may be able to, I will ask him.
.

Hey buddy, have you had a chance to ask your fabricator about the rear ladder? I want one just like yours, and then would need to be shipped. David

 

[sean257]

Hey buddy, have you had a chance to ask your fabricator about the rear ladder? I want one just like yours, and then would need to be shipped. David

Sorry for not getting back to you David. He does not work at the fab shop anymore.

 

[sean257]

I have a new problem that I am sorting out. The truck just dies sometimes after idling, and sometimes will not restart. Sometimes it runs after I have been driving, shut it off, and then start it back up - it will run rough just after restart for a bit.

Before the 8.1 went in, we changed the crank position sensor. From what I am reading, it sounds like the crank position sensor could be the problem again. What is happening is that the fuel pump relay is not always telling the fuel pump to run. The fuel pump will run when jumpered, but it seems that the green wire with white stripe from the PCM to the relay is not always sending signal. I have not found in detail exactly what the CKS does when failing other than it makes the motor not run (ie, I have not seen any details if the failing CKS makes the ignition system or fuel system stop the motor)
When the Sensor was changed, it was replaced with a CARQUEST part No 71-4868, which appears to cross reference with a GM 12575172 which I believe is the correct replacement CKS. Any thoughts or opinions would be greatly appreciated.

All and all the truck has been good for a year now. I slid sideways a bit down a hill and damaged the passenger side, but it is just cosmetics.



A snow picture from before I smashed the passenger side

 

[Larry]

That is a bummer to hear Sean but what you describe could very well be a failing crank sensor…. and about a 100 other things all the way down to a 4L80E torque converter clutch issue. Have you pulled codes to start some strategy based diagnostics before starting to just throw parts at the issue? Because that crank sensor is such a SOB to replace I would have recommended only using a GM or AC Delco (Delphi) part. If the crank sensor turns out to be the culprit it looks like the engine will need to be moved forward a few inches or removed completely to gain access to it as it is doubtful it will come out due to interference with the firewall on these old trucks. Believe me, I FEAR the day the 8.1L crank sensor ever fails on my K10 or Polar Bear Suburban!

FWIW, when the crank sensor failed on my 2001 Silverado it started dying at stop lights once in a blue moon. Then maybe twice a week, then twice a day then it died in traffic and never started again until the sensor was replaced. That truck had the early design 2001 sensor that shot craps at 48,000 miles which the 2001 and 2002 sensors had pretty much a 100% failure rate where GM even recalled them. I have not actually heard of the later design sensors failing but if you used China’s finest auto parts (who knows who CarQuest uses as the manufacture of their house brand but it is unlikely that it is an upper spec limit Delphi OEM piece) the sensor could definitely be suspect.

One thing I’ve noticed on failed crank sensors on any late model GM engine is there is no tach movement during cranking on the tachometer itself or on a scan tool tach readout. The tach will just be dead at 0 RPM whereas, typically the tach wiggles and hops during cranking. That is not scientific, that is just a personal observation.

WTH did you do to the body on that poor thing? Just cosmetic my foot! That looks like a pretty serious mishap.

 

[sean257]

I did notice that the scan gauge that I keep plugged in registers RPMs while cranking. The biggest thing is the green wire with the white stripe coming from the PCM to the fuel pump relay is not telling the fuel pump relay to turn on the fuel pump, but I do not know why. I also have a temp sensor that has gone bad and is being replaced, but, again, I have no idea if that would make the PCM not send signal for fuel.


We drive through trees sometimes. this past summer I was going through a trail that I carved out with the little CJ7 and the Suburban had to perform a considerable amount of trail widening. At one point I was ~45 degrees to a side incline dropping off to the passenger side/rear and I had two trees at the front bumper that I could not push down. I foolishly turned on the rear locker to help push, which slid the back in the passenger direction and into a couple of large trees, that caved in the door and smashed the window. It wasn't force from speed as much as just the weight of the truck that imploded that side. It also locked me in with a large tree behind me that I could not push down either. I gave one last attempt to muscle our way out forward, promptly breaking the rear drive shaft off at the transfer case end. I ended up cutting down three trees in front of me with my chainsaw and, using the winch and front wheel drive, got free. Once free I had to remove the rear driveshaft so I could drive home.

 

[Larry]

But no codes? Have you checked spark? Does it build fuel pressure (energize the fuel pump relay) when the key is first cycled on (key on/engine off)? If the ECM is not seeing an RPM signal it will not energize the pump other than the first few seconds during initial engine crank. Another thing you might try doing is trying to run the engine on an alternative fuel source such as hooking up a can of injector cleaner to the fuel rail port to see if it will run on that. If it does, then that would point to something causing the ECM to not send a signal to the fuel pump relay. If it doesn’t, then that would point to an ignition problem such as a crank sensor. It’s very unlikely it is an ECM itself. Those ECM’s are pretty much bullet proof unless the power going to it is stretchy where it could have been zapped. I believe you are running a Howell Harness?? If that is the case, give them a call and pick their brains as well. Good luck!

Bummer about the body damage.

 

[sean257]

Thanks again Larry! It looks like I had a bad connection on main power going to the ECM that was causing the problems. Between that and the temp sensor all seems to be well.

I am not too worried about the dents, but I may replace the door at some point so I can have a window that rolls up and down again.

 

[Larry]

Glad it turned out to be something simple. Those damn power and ground can cause all kinds of weird problems if they aren't clean and securely connected.

 

[sean257]

Been a while since I've logged on here. I've had the Suburban at a shop try's to figure out a rough idle slow speed problem for quite a while now (they work on it when they can). New fuel pump, pressure is good, spark good, new computer, but the problem remains. They are going to run it on a dyno while hooked to the computer to see if they can finally figure it out. If that weren't frustrating enough, some POS stole it from out front of the shop last week. Long story short, it has been found and I have it back, but without my front bumper, winch, stereo, lights, and all of my recovery stuff that was in the back.

Now that I have it back, I am considering fixing the body and painting it white (I really like Larry's polar bear) or grey, with black wheels. I'm wondering if I'd be just as well off to just find one the color I want and putting that body on my chassis...

My daughter is strongly opposed to the idea, because she likes the character of the dents. If she wins we will just get new flares (3 out of 4 are half missing) and bumper, lights, and stereo.