1990 Suburban acting up - won't start at times

[97TurboAstro]

I know this isn't Astro related, but it is TBI-related.

Anyone have any ideas?

from http://www.fullsizechevy.com/forums/gen ... times.html


Some good friends of ours have a 1990 1500 Suburban 2WD. Odometer reads 266k miles. Truck got a new engine about 25k miles ago.

Problem
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Truck was mysteriously dying once every 4-6 months. It would leave 'em stuck on the side of the road. When they towed it to the repair shop, it would run fine.

About six months ago, the shop installed a new ignition module.

June 13th, it died in the middle of Dallas traffic. It was towed to the repair shop. They kept it for the entire next week. Drove it all over the place. It never died.

I told them, and they understand... you can't find the problem when it's running okay.

June 20, on the way to a wedding, it died. Owner was NOT happy.
They sat on the side of the highway for about 45 minutes... waiting for me to come look at it. By the time I got there.. you guessed it... the truck started right up. After the wedding, I took the truck home. I told them I'd look at it.

The next morning, I drove the truck to church. It croaked right in front of the church. <yippie!> As usual, the truck would crank just fine, but wouldn't fire. I shot a couple of squirts of starting fluid in the throttle body. It started and ran for a few seconds. I did that several times, and declared the problem to be the ancient fuel pump.

After church it had coolded down, so I took the truck home and drained the 40 gallons of fuel into various containers. That took me a while, because my Haynes manual told me that the fuel pump test wire was in pin G of the ALDL connector... that slot is empty.

I found a red wire hanging off what I was guessing to be the fuel pump relay. I ran 12V to that wire, and the fuel started flowing. I managed to drain the entire tank without the pump stoping... that should have been a clue that the pump wasn't the problem.

This morning, I dropped the tank, swapped the pump, and put all that fuel back in the tank. I then decided to drive the truck to work... just to prove to myself that I had fixed it [patting self on back]. I didn't get five miles from home when the SES light came on and the truck coasted to the side of the road. I was not happy. BTW, it was about 98 degrees out by then.

I didn't have any tools with me. I took my pocket knife and wacked the fuel pump relay a couple times. The silly thing started when I try again. I drove it about 1/4 mile, and the truck died, again. I tried a few times to get it re-started. I lifted the hood, and smacked the relay a couple more times. It started right up. I drove another 1/4 mile and it died. I was now pretty sure the problem was that blasted relay. I got out... <tap.. tap... tap>... it wouldn't start... what? I called my wife to rescue me. I had her bring the wire I was using for the fuel pump test port.

I put 12V to the test lead... wouldn't start.
I pulled the wire connector from the relay and put 12V to the grey wire... wouldn't start

I gave up...

Any ideas??

I guess I really need to check fuel pressure and see if the pump is running. If the pump runs, but fuel pressure is low.. that probably means the fuel pressure regulator, right?

If I have good fuel pressure.. that means the injectors aren't spraying fuel. Would that be the computer or injectors?

I'm getting quite upset with this truck. I haven't even tried to start it since I got home... it'll probably start right up... <sigh>

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Just went outside... truck started right up.

Would a bad oil pressure sender do this?

How/where do I easily hook-up a temporary fuel pressure gauge?

Is the gray wire on the fuel pump relay harness the wire that goes directly to the pump? If so, that would imply that the problem could be the ECM... maybe?

What should I take with me for my next hope-it-fails trip?

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When "it" happens, the truck doesn't seem to lurch and surge as if it's running out of fuel. It just stops running and the SES light mocks me from the instrument cluster.

Since the SES light was coming on, I just pulled the codes. I only got 12s, so that was no help.

[dunedog]

I guess I would look at the simple stuff 1st.........is the coil wire good? replace it anyway if it hasn't been.

[97TurboAstro]

Since the truck will start with a squirt of starting fluid, I don't think it's an ignition problem

[LiftedAWDAstro]

If you can throw a fuel pressure gauge on her, see if you are getting fuel. If there is fuel in the rail but still no start, I would look to the ECM. If no fuel then start checking every connection from the ECM back to see if there is voltage to the pump. Check fuses for one that is loose or a loose connection at the fuse block. Time to get the meter out!

[CaptSquid]

Give it a shot of ether before that. If it starts, then it IS the fuel delivery system. If it STILL doesn't start, then it's the ignition system. Check for sparks.

[97TurboAstro]

I did some more testing this afternoon.

I put together a fist full of parts to get a fuel pressure gauge on the system. When the engine is cold, the fuel pressure was about 11 psi. As soon as I shut off the engine, the pressure dropped to zero, but I think that's the way TBI is setup.

I did some testing on the fuel pump relay. I started the engine, and then unplugged the connector on the fuel pressure relay. The engine continued to run. That means the oil pressure switch is working okay.

I think started the truck and just let it sit and idle. After the engine warmed-up, I noticed that the fuel pressure had dropped to about 10 psi.

The truck just sat there and idled just fine. I then got in and held the throttle open for a while. After about 20-30 seconds of 1500-2000 rpm, the SES light went on, and the engine quit. That was great... 'cause I was still in my driveway.

I quickly took my test lead and put power directly to the fuel pump relay. The pump buzzed, and the pressure jumped to about 10 psi. I had my wife try to start the truck. It wouldn't start. I removed my jumper wire.

I removed the cover to the air cleaner, and gave the throttle body a shot of starting fluid. My wife tried to start the engine. It fired-up, then the engine died, again. I didn't get a chance to look at my fuel pressure gauge.

I gave it another shot of ether. My wife cranked the engine, and it fired right up. The fuel pressure came up, and the engine continued to run... for several minutes.

I had my wife hold the throttle open to see if it would die... of course it didn't.

I then noticed that the fuel pressure was only about 9.5 psi... and it didn't change when I raised the rpm.

I decided to turn it off and try again, later after the engine coold-off. The dropping fuel pressure is odd.

I noticed that the ECM does not run the fuel pump for a couple seconds when I first turn on the key. Is it supposed to on a TBI setup?

It's really odd that I can enable the fuel pump... bring the fuel pressure up, but it still won't start. To me, that implies that the injectors aren't firing... hmmm.
But.. when I sprayed starting fluid it started... so that implies the sparks are there... just no fuel

After the 2nd squirt of fuel, the computer suddenly decided to enable the fuel injectors...

Any comments? I'm running out of ideas...

May be time to track-down a cheap ECM??? I think i'ts a 1227747

[6spd_monte]

I noticed that the ECM does not run the fuel pump for a couple seconds when I first turn on the key. Is it supposed to on a TBI setup?

On my van the fuel pump runs for about 3 seconds when the key is first turned on, and I have TBI too.

[97TurboAstro]

Since my last update, I broke-out the volt meter and did some pokin' around.

When I turn the key on, the trigger wire to the fuel pump relay runs for a few seconds... and the fuel pump runs, so I know the ECM is trying to turn on the pump.

I pulled the ECM from under the dash and probed the connectors at the ECM.
All the 12V pins have 12V.
The fuel pump signal has 12V
Injector A and B have 12V each

If I touch D14 or D16 while it's running, I can hear the engine idle change, so I know both injectors are firing.

If I put a test light on B5, it kills the engine... oops.

When the truck runs, I see about 1.5V on B5 with my VOM.
When it won't start, I see less than .1V on B5.

I thought it might be the ignition module. I pulled the old one. I noticed it didn't have the high-temp compound on the back, so I thought the heat could be messing it up. I took it to Auto Zone and they tested it about 12-15 times in a row. It passed every time. I bought some of that high-temp compound.

I re-installed the ignition module with the high-temp compound, let it idle for 15 min, then drove it around the block. I got almost all the way around the block and it died. I figured the ignition module was bad...even though AZ said it was okay.

I bought a new ignition module. It came with more high-temp compound. I installed it... let it idle... then took it for a ride. I got around the block once... but not twice I was even able to stick my foot to the floor once or twice.

During all my testing, I noticed that the truck will crawl along at 10-15 mph, but if I step on it... that's when it'll cough/sputter/die.

I've run out stuff to "fix"....

Could this be an ECM thing?.. injector drivers overheating?

Anyone have any ideas?
What else can I test w/ a simple VOM?
If I borrow an o-scope from work, would that be able to tell me anything?

Anyone in Dallas area have a 1227747 I can borrow?

[rlsllc]

I went through this with my '91 Astro over a year ago, and it was the ignition module. V8Famvan (James) gave my the test sequence, but I lost it when my computer crashed. Should be on the site somewhere, if you want to verify that your old one was bad.

The ignition module also tells the ecm when to fire the injectors, or something like that.

As for the "cough/sputter/die", I have no idea, mine did all the other stuff you described, but when it ran, it ran fine.

After the new module, never a problem for over a year, then it just let go one day.


Edit: here is a link to when I had this problem, the test is in there viewtopic.php?f=4&t=1260&start=0&st=0&sk=t&sd=a

[v8famvan]

Ron, Sounds like you got a good one there.
From what you have posted it sure sounds like an electronic device is breaking down/failing with heat. From my experience, it sounds like an ignition module issue but you have replaced that more than once. (Are you sure the terminals are ok to the ignition module?) The next likely failing device is as you suspect, an ECM.

As mentioned by Roy, the test in the thread he linked to should help you test the computers ability to respond to a 'simulated' distributor signal. The injectors should fire & the fuel pump should be triggered to run.

Also note this: Yes, the ECM should run the fuel pump relay/fuel pump for 2 seconds after the key is turned on. This is a programmed response. The fuel pump should also be triggered to run again when it recieves an ignition trigger signal. So, turn the key on, 2 second prime. Tick the engine over (from the ignition ON position) & the pump should run again. If it does not, this is not normal

[97TurboAstro]

I finally got winALDL working on the Suburban.
Andrew at AKM Cables was a great help.

All the ALDL data looks great when the truck
is running.

When it dies, MAP, TPS, and all the usual stuff
looks fine.

But, I can't get any RPM values when cranking.

I pulled a spark plug wire and put a plug on it.
I don't see a spark most of the time.

I swapped-out the coil for the coil from my Syclone.
Still no spark or RPM values.

I still have the brand new ignition module installed.
Could this be the distributor?

The truck has 166,000 miles on it.

Ron

[6spd_monte]

It could be the pickup coil in the distributor, but I don't know how to test it...

[v8famvan]

As a general rule of thumb in our shop, we replace the pick-up coil whenever we replace an ignition module.

No spark is not caused by the computer, only the primary ignition system (distributor shaft, pick-up coil, ignition module, ignition coil, wiring between them) & the secondary system (ignition coil, rotor, distributor cap, wires & plugs).

[97TurboAstro]

Tonight I replaced the pickup coil, but didn't fix the problem

I tested the resistance of the new pickup. It was about 840 ohms. I put the new pickup coil on the intake manifold, so it would warm-up. I started the truck, and waited for it to die... about 15 minutes later... it died. I checked the resistance of the new coil after it had warmed-up.. it was about 930 ohms. I quickly removed the dist cap and check the resistance of the old coil. I was about 950 ohms or so. I pulled the distributor and changed it, anyway.

It appears to run better, but it still dies. <sigh>
I was able to drive it around farther this time, but that may have been because it was rather cool this evening... only about 85.

After it died this time, I was so confused, that I drug my wife outside to help.

I started from the top...
I had her turn the key on... what? no fuel pressure...?
I had her turn the key off and on a couple times no <click> from the relay.

I ran into the shop and grabbed my jumper wire. I jumped the battery to the red wire on the relay, and the fuel pump came on as usual.

I had her turn the key off and on a couple more time. Each time she turned it on, the relay would click and the pressure came up to normal.

The one thing I didn't check when the relay did not come one was if the ECM was telling the relay to come one or if it was just sitting there. <drat!>

But... a bad relay still shouldn't have killed the engine... because the oil pressure switch is a backup. When it was running last week, I tested the oil pressure switch by unplugging the relay while the truck was running... didn't die.

I want to blame the ECM, but I'll have to wait 'til tomorrow evening...

So.. if when it won't turn on the relay, the ECM isn't sending the signal... that pretty much has to be the ECM... right? The ECM doesn't check for dist signal when you first turn on the key... does it?

Getting *REALLY* tired of this crazy thing...

[kings-x59]

or, the wire from ecm to relay...