What's the next step ???

[MEdlin]

My 94 Suburban with the 6.5TD (F) engine and 182,600 miles is about to get the best of me!

In a quest to solve a problem with random stalling, I did the following:

- Check for Codes, just DTC 12
- Replaced Fuel Filter
- Began using Stanadyne Fuel Additive
- Replaced Electric Fuel Pump
- Replaced the 5 amp fuel pump fuse in the fuse block below dash on driver's side (don't know why it was blown)
- Replaced Oil Pressure Switch
- Checked and cleaned all connectors and grounds
- Replaced Electric Fuel Pump Relay

Everything seemed great until I was 2200 miles away from home while on vacation!
The stalling problem reared it's ugly head several times again, but the truck always restarted. Never got a DTC or SES light.

The GM Dealership in Golden, CO had the truck for two days and diagnosed the problem to be the PCM. They replaced the PCM and EPROM, and set the timing for higher altitudes. 150 miles later on the way to Utah, the truck stalled again. The stalling typically occurred soon after starting the engine (cold and hot), sometimes several times, but would eventually run fine. Still no codes. Called the mechanic in Golden and he suggested removing the electrical in-line filter before the Optical Sensor on the Injector Pump. I did, but there was no change.

When I got back home, I replaced the FSD and Resister (#4), but mounted it on an external cooler similar to the BETA FSD cooler. I left the old FSD in place because I didn't want to disturb the timing, or go through the process of removing the Intake Manifold (been there, done that). I continue to monitor the FSD temperature and its within operating temperature as specified by Stanadyne ( -40 to +240 F).

Truck seems to be running fine for a day or two, until one evening after work it will not start. I notice no Glow Plug light. When turning the key to ON, during the Lamp Test process, the Service Engine light is supposed to blink once as part of the PCM self-test. I noticed when the Glow Plug light did not come on, the SES light does not blink. When I check for codes following this scenario, the SES light just stays on, no blinking / no codes.

I replaced the Ignition Switch and Glow Plug Relay.

Now, sometimes the truck will start, sometimes it won't. If I turn the key and don't see the Glow Plug light, there's no use trying because it won't start.

While replacing the Glow Plug Relay, I could force a DTC 29 by unplugging the connector from the relay. Otherwise no codes.

I have to believe this is due to another relay somewhere, a short, or a lose connection. I just don't know where.

Any ideas?

[MTTwister]

While your waiting for a more definitive answer - Check, Clean and tighten all grounds? Rear pass head, frame, etc? Oh and Good Luck

[MEdlin]

MTTwister,

Already did that. There were approximately 3 grounds on the back passenger side of the engine, plus others along the intake manifold. I'm wondering if there could be something loose around the PCM where the dealer replaced it.

Thanks, I'll need all the luck I can get. ;)

[charliepeterson]

With the engine cold unplug the coolant temperature switch. This will default to -40* and keep the glow plug light on much longer. If the glow plugs don't come on she won't start very easily.

The ECM B circuit in the under hood fuse box is what gives power to the Injection Pump. It's too bad the dealer didn't know what he was doing on the truck.

A hot, tired PMD will turn off the truck like the key was turned off and never turn on the SES light. Ignition switch circuit can do this too but less common.

[MEdlin]

Charlie P.

My problem now seems to be with a hot start. I removed, cleaned, adjusted, and replaced the pins from the connector that plugs into the Glow Plug Relay. I thought I was having a problem in this circuit. Note, the Relay and Ignition switch are brand new, as are the ECM, EPROM, and FSD. After reworking this connector, the truck started great for several times in a row. I took the truck for about a 40 mile drive to recharge the batteries (been in the garage for 2 weeks). When I returned home, the truck would not start after I turned it off. My engine temp was 190*-195* F and my external mounted FSD was about 176*F.

Thanks for your suggestions. smile.gif

Mark

[CareyWeber]

I think I'd look at the ignition switch, and check all wiring (look where the harrness going into the cab to the ecm).

Carey

[DickWells]

MEdlin: As a shot-in-the-dark, you might want to try this.
On the right hand side of the IP, there's a bubber block buried down in there with (I think) six wires through it. Could be eight, I've forgotten. Anyway, you can fish that block up high enough to work on it. It's about 1" + wide, by maybe 2" long, and 1/2" thick. I had a bad wire through mine, that gave me all kinds of trouble. I was lucky, and found it quite quickly, after I had replaced my FSD. I probed each wire both sides of the block, with sharp probes and an OHM meter. When I found the bad one, I just jumpered around the block, and the Sub. ran a whole lot better after that.
The block is a noise filter, or some such thing, but I had no ill effects from by-passing it.
Just hoping that you have something simple, like this.
Good luck.
Dick Wells

[catmandoo]

dick i was at the ac delco parts house the other day,looking for a fuel pump relay and the only thing he could come up with in his book was a fuel pressure regulator and when he took it out of the box it was that thing your talking about.had a harness on each side,?? i thought the press reg was on the pump myself.or maybe thats just the fuel solenoid!!!

[MEdlin]

Carey,

I did change the ignition switch, but I didn't pay close attention to the connectors. I'll have to take another look.

Dick,

I had a GMC mechanic tell me to remove the noise filter. This plugs between the Optical Sensor and the original plug. I took this off in an effort to solve a problem with the truck quitting. Dealer replaced ECM (PCM). As I think about it, since I replaced the FSD, I have not had this filter installed. I'll give it a try.

The strange thing is, when I turn the truck off, if I don't hear the Fuel Lift Pump running for a few seconds afterwards, I know it won't restart and the Glow Plug light won't come on.

Again, this is usually when the truck is hot.

I did look at the place in the firewall on the passenger side where the wiring harness goes through from the engine. The wires were going over the top of an aluminum AC pipe. I didn't see a problem, but did put a piece of heater hose between the wires and the AC pipe.

Thanks for the ideas. smile.gif

-Mark

[DickWells]

Well, that "bubber" (actually rubber) block, is probably a noise filter, and since your truck ran without it, before, I assume it should run without it, now. I guess I'm stumped, regarding your problem. Not of much help, I'm afraid.
I just happened to think. Though it seems very unlikely, when I changed my electronic DS4 to a mechanical DB2, I couldn't start the Suburban. Well, apparently, the lift pump had quit before I took out the DS4, and I hadn't pulled it really hard, so I didn't know it had gone South. Anyway, the point I'm trying to make is that there are many possibilities for more than one thing being wrong at the same time. All those lousy grounds in the truck are so important, that it can take a lot of time tracing them out, just as an instance.
The one at the back end of the passenger side intake manifold, stopped my rig dead, after it quit intermittently over a week-long period. The one on top of the frame, next to the starter, did the same thing. I fixed that one (actually two flat, copper straps under the same bolt) by running a 5/16 drill through the top of the frame from behind the tire, and putting in a 5/16 X 4' SS bolt and double-nutting the terminals there.
Catmando: are you looking to put a relay in to take the load off your oil pressure switch? I bought mine from Advance Parts for about $3. That was close to 3 years ago, with no more switch problems to date. The switch I took out at that time, had the contacts inside burned to a crisp.
Keep in touch.
Dick

[catmandoo]

hey dick i am kinda leaning towards a relay i think it's gonna be goin on before to long,3 bucks sounds good you got a part number??

[tom.mcinerney]

Mark-
These may be 'point of departure':

1.)>>The strange thing is, when I turn the truck off, if I don't hear the Fuel Lift Pump running for a few seconds afterwards, I know it won't restart and the Glow Plug light won't come on.

2.)>>A hot, tired PMD will turn off the truck like the key was turned off and never turn on the SES light. +
>>The ECM B circuit in the under hood fuse box is what gives power to the Injection Pump.

3.)>>check all wiring (look where the harrness going into the cab to the ecm).

I'd check [wiggle/inspect, but renew regardless] the ECM B fuse.
Recall that a few members have had new FSDs fail within short time...also a few found that bad connector at FSD required replacement or tweeking.
A dying FSD can short and deprive LP of juice, since in same circuit.

Carey's suggestion of harness troubleshooting may be in order. His good work locating a short in the AC wiring is consistent with Helm troubleshooting,though most of us don't get that far.

-best of luck-

[rickmeyer]

how about replacing the oil pressure sender. Im assuming on the DS4 equipted truck this is also a oil pressure cut-off switch, shutting the engine down in the event of a loss of oil presure. Next time it wont start, sheck to see if you have power to the fuel selonoid.

[JohnC]

Originally posted by rickmeyer:
Im assuming on the DS4 equipted truck this is also a oil pressure cut-off switch, shutting the engine down in the event of a loss of oil presure.Nothing that fancy: just power to the lift pump.

[DickWells]

Hi Katmando: Sorry I haven't been on here for many days.
Can't tell you a part number, but if you look for a relay that will hold, say, 10 amps, you'll be OK. Mine is a little black, square one. Maybe a one inch cube. Should be able to pick one up at any parts store. It's been in there for maybe three years now, give or take. Probably 60K miles.
Anyway, you've probably got one by now.
Good luck.
Dick