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Jeremy airjer W

Who's up for a diagnostic challenge?

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Today I had a 2005 Chevy Express with a 4.3 and approx. 150k miles waiting for me when I arrived. The work order said it was towed in the night before after it stalled on the freeway the customer noted that the gauges had gone dead and some but not all the warning lights came on BEFORE it stalled. They also mentioned that the ABS light would always stay on.

Lets walk through this in a step by step manner. What should we do first?

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Hook up an obd scan tool and clear any codes, Unhook the battery and then hook it back up and start the car if it will start. See what codes come back up. 

During that process check the battery and terminals to make sure you are getting proper voltage to the computer and instruments.

Probably a blown fuse,bad CPU,faulty ground, connection,battery...

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step one. Confirm the customer complaint.

I attempt to start the vehicle and it starts normally. I think to myself, "this is going to be fun!" Gauges are working correctly, the ABS light is on, and it is running just fine. I just ruled out the battery, connections, and alternator as the battery should be dead if the alternator had failed.

I let the vehicle run long enough for the temp gauge to read about a quarter of the way and sure enough the gauges start going wonky, the security light comes on, and the check gauges light comes on. as soon as the gauges stop fluctuating the lights turn off accept for the ABS light. Eventually the vehicle stalls but starts right back up and the scenario replays. Eventually the engine stalls and will not restart at this time the temp gauge is nearly at normal operating temp. 

At this point I have enough clues to start making some theories. The clues are runs fine until the engine starts warming up, gauges fluctuate/go completely dead, only the ABS, security, and check gauges warning lights illuminate. Engine will not restart once it is hot.

Whats the next step?

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Since it runs fine until it gets up to speed I would guess that is either a problem with a sensor or CPU. It seems to be too many things not related to each other to be anything except an electrical issue and more than likely in the CPU. 

 

At first with the gauges I was thinking clock spring but if that failed it would give nothing ever.

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Actually I think it runs fine until it gets hot, up to normal operating temperature.  

Some kind of breaker or thermal switch?  Not that I know anything about cars newer than about 1970, when it comes to fixing them. 

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After It would not start I needed to wrap up some other jobs so it sat for about 1/2 hour. I decided the next step would be to what codes where present. The scan tool has an option to check for codes in all of the computers in one shot so i started with that. I had a code for EBCM communication as well as a code for class 2 data communication. Well that didn't help at the time but made more sense later on.

I had a bay open and decided to see if it would start to pull it in, and it did. I pulled it into the shop and again let it run until it stalled and would not start. I had a sneaking suspicion that the problem was fuel related and a quick shot of carb spray in the snorkel and it fired and ran for a short time and then would not restart. I decided to pull the doghouse and check for fuel pressure. There was none! Aha, I might be on to something but that still does not explain the dash and warning lights.

At this point I am starting to hone in on a single theory that I have run into in the past with similar symptoms. the next step will confirm it.

Whats the next step?

 

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39 minutes ago, leech~~ said:

Fuel filter and pump. Maybe mass air sensor?

 

36 minutes ago, PurpleFloyd said:

Vapor lock

Neither of these would cause the gauges to malfunction....

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1 minute ago, rl_sd said:

Just because the stalling is fuel related, doesn't mean that it can't still be electrical in nature due to components within the system. Was pressure checked before or after the fuel pressure regulator? The regulator, MAP, and MAS all give and receive feedback to the ECM... still banking on a computer or main harness connection issue.

I think your both right from what he is saying. It must be an electrical issue causing the gauges to malfunction and shutting down the fuel.

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16 hours ago, Jeremy airjer W said:

 

Neither of these would cause the gauges to malfunction....

True, I was thinking it more if an effect than the cause. 

I am still leaning towards it being related to sensors/ CPU/ECM. 

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Think simple......

We know we have a lack of fuel pressure but it can't be a simple as a fuel pump. What can we check/test to see why there is no fuel pressure?

In reply to the MAF, MAP, and regulator. The regulator gives no feedback on this engine. When cranking the fuel pump is powered up regardless of the MAP, MAP signal. That rules all three of those components out of the equation.

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I am going to guess is has something to due with temp or something getting too hot and causing an issue. Once it cools it goes away until things warm up again but I have no idea what lol

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1 hour ago, Jeremy airjer W said:

Think simple......

We know we have a lack of fuel pressure but it can't be a simple as a fuel pump. What can we check/test to see why there is no fuel pressure?

In reply to the MAF, MAP, and regulator. The regulator gives no feedback on this engine. When cranking the fuel pump is powered up regardless of the MAP, MAP signal. That rules all three of those components out of the equation.

Low battery voltage!

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I am going to go out on limb, and say as the engine warms a fuel line of some sort or sensor is constricted, be it by shrinking or swelling due to the heat.

Thus causing the a fuel pressure "issue", triggers the engine to go into limp mode and then consequently shutting the motor down, and triggering the instrument panel to light up.

Keep in mind, I know nothing of vehicle mechanics.

Edited by ANYFISH2

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4 hours ago, leech~~ said:

Low battery voltage!

Close enough :grin:

Anytime we dont have fuel pressure we checked for power and ground as close to the fuel pump as possible. The one simple test we can do is check for power to the pump

Fortunately the harness to the pump from the main harness to the rear of the vehicle is easily accessible. I connect a voltmeter to the power wire to the pump and turn the key on. I have 12 volts.......... wait for it ............... then roughly 3 seconds later 8 volts. I continue to have 8 volts until the key is turned off. Voltage then drops to 0 (zero).  Fuel pressure gauge still reads zero.

AHA, problem solved and I know exactly where to look.

Anyone want to take a stab at what that one simple test told me?

 

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Out of Gas  see i did not cheat and read ahead.

Edited by jmd1

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43 minutes ago, delcecchi said:

Why would that be affected by engine temp?

I'm glad you asked.

I knew exactly where to look for this ground strap. One end should be bolted to the frame rail on the passenger side near the upper control arm. The other end is bolted to an accessory bracket on the front of the engine where the main ground from the battery attaches. Key work is "SHOULD"

Much to my surprise the bolt and eyelet where there but the strap was not. Search a little with a flashlight and it had been relocated to the frame cross member that runs in between the front suspension components. It looked like it was in good shape from what I could see from the top so I raised the vehicle to see the rest of it. Again it was pretty corroded but in tact. The curious part was that it was resting/touching the lower radiator when it was relocated. I grabbed it and gave it a little tug and it essentially disintegrated.

Heat = resistance. With a cold engine the strap was able to sufficiently function. When the engine warmed up the resistance rose to the point that it was no longer a conductor but a load on the circuit. Resting on the lower radiator hose was not helping.

I replaced it with an 18" battery ground cable and it was back on the road.

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