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2000 C2500 stuck in closed loop.


20GMC00
03-03-2016, 10:49 AM
I have an old body style 2000 C2500 with a 5.7. It has been getting harder to start. It has a new Delphi fuel pump and new battery. I need to crank it 4-5 times before it will start sometimes. Sometimes it won't start at all. I found out that if I put a 10 amp battery charger on it for just a few seconds, it fires right up. Also I have my live data scanner plugged in and it is staying in closed loop all the time and both rear O2 sensors are staying at 99%. It runs fine but it's just getting harder to start. Has anyone ever had this happen or have any ideas? Thanks!

j cAT
03-03-2016, 11:09 AM
I have an old body style 2000 C2500 with a 5.7. It has been getting harder to start. It has a new Delphi fuel pump and new battery. I need to crank it 4-5 times before it will start sometimes. Sometimes it won't start at all. I found out that if I put a 10 amp battery charger on it for just a few seconds, it fires right up. Also I have my live data scanner plugged in and it is staying in closed loop all the time and both rear O2 sensors are staying at 99%. It runs fine but it's just getting harder to start. Has anyone ever had this happen or have any ideas? Thanks!

if the scan tool reports the wrong cold start temp then that is the problem

20GMC00
03-03-2016, 12:33 PM
When should it go out of closed loop?

20GMC00
03-03-2016, 12:53 PM
Looks like my fuel pump is taking a dump already. 50psi with key on and 60-62psi the second I put the battery charger on it.

20GMC00
03-03-2016, 01:13 PM
I just took the filter off and the stuff that came out the inlet of the filter looked coffee.

777stickman
03-03-2016, 02:54 PM
When should it go out of closed loop?

The system is in "open loop" when first started cold. The system generally goes into "closed loop" when 1) the o2 sensors get warmed up, 2) the ECT sensor reaches a specific temp, 3) after a specific amount of time from first start.

During normal operation the VCM will stay in closed loop and fuel is controlled by the o2 sensors.

j cAT
03-03-2016, 05:18 PM
When should it go out of closed loop?

at about 160deg f plus.

j cAT
03-03-2016, 05:20 PM
I just took the filter off and the stuff that came out the inlet of the filter looked coffee.

some one may have put crap in the tank. you will need to drop tank and wash it out. show picture .........

20GMC00
03-03-2016, 05:49 PM
I will do that. Thanks.

20GMC00
03-08-2016, 04:43 PM
It starts right up with the fuel return pinched off. Is the regulator replaceable or is it part of the spider? It's got 268,000 miles. I'd say it's about due.

777stickman
03-08-2016, 06:00 PM
The regulator is replaceable on its own. It's under the upper manifold.

j cAT
03-08-2016, 06:23 PM
It starts right up with the fuel return pinched off. Is the regulator replaceable or is it part of the spider? It's got 268,000 miles. I'd say it's about due.

probably returning too much fuel . driver side of the intake manifold.

20GMC00
05-23-2016, 10:58 PM
Finally had time to drop the tank tonight and it is still spotless on the inside. I'm guessing the pump went to crap causing the filter to be full of the black stuff.

harleyguy
05-24-2016, 01:27 AM
To me it sounds like your having a battery/electrical problem putting a charger should not boost your fuel psi.
As for your closed loop problem what are you showing for ect. As stated that is one thing that tells it when to run closed/open loop.
Check for 12.6v on your battery and also to your fuel pump

20GMC00
05-24-2016, 08:47 AM
The voltage is good. I put the charger on 10 amp and it will start within 10 seconds after putting the charger on.

Schurkey
05-24-2016, 10:47 AM
You cannot diagnose fuel pump pressure problems until you know the voltage drop of the wiring that powers (and grounds) the pump. Under-sized wiring used by GM on some vehicles drops pump RPM significantly. If that's the case with your vehicle, it will be VERY intolerant of corroded connections, or corroded contacts in the pump relay and oil pressure switch.

If the pump has broken-up mechanically, so there is pump debris in the filter...fine, you need a new pump. You certainly need a new fuel filter. Diagnosing fuel pump pressure with a plugged filter is a waste of time, and leads to incorrect diagnosis.

"I" will not install a pump without looking at the pump voltage, (measurement taken as near the pump as practical) ground-side voltage drop, and pump amperage draw. Amperage draw is best looked-at using an amperage probe and an oscilloscope. With a digital oscilloscope, I can verify the amperage draw of each individual armature bar in the pump motor, and also determine the pump RPM.

20GMC00
05-24-2016, 11:14 AM
Ok. I'm just telling you what's going on.

20GMC00
05-24-2016, 11:23 AM
And I don't remember saying that I was trying to diagnose fuel pump pressure with a plugged filter. I change fuel filters more often than I need to.

Schurkey
05-24-2016, 01:52 PM
And I don't remember saying that I was trying to diagnose fuel pump pressure with a plugged filter. I change fuel filters more often than I need to.
No problem. Let me remind you.
Looks like my fuel pump is taking a dump already. 50psi with key on and 60-62psi the second I put the battery charger on it.

I just took the filter off and the stuff that came out the inlet of the filter looked coffee.
Based on your own words, and the times that you posted, you were testing fuel pressure before you found the icky stuff in the fuel filter.

But that's ok. I can walk away from this. Unsubscribed.

20GMC00
05-24-2016, 01:57 PM
Your right, I did check it that one time. A guy shouldn't have to worry about the filter being that dirty that quick. I got it fixed. Don't worry about it.

maxwedge
05-24-2016, 03:17 PM
I believe this truck has heated o2 sensors, so coolant temps are not part of the parameters needed for closed loop, see if they have 4 wires.

20GMC00
05-24-2016, 03:30 PM
You are correct. They have 4 wires.

rhandwor
05-24-2016, 05:26 PM
Hold the volt meter on the battery. Have someone else try and start the car.. A battery can have proper voltage and when loaded drop way down.

777stickman
05-24-2016, 06:39 PM
Concerning the questions of OPEN vs CLOSED Loop. This is from a '99 Old Body Style service manual. Yours is an '00 Old Body Style so I would think it would be the same? This manual also shows the 4 wire HO2S.

j cAT
05-25-2016, 08:27 AM
The voltage is good. I put the charger on 10 amp and it will start within 10 seconds after putting the charger on.

what is important is the power at the pump. most times these trucks have new pumps installed this is then a wiring screw up when the repairer does not solder in the new connector to plug in to the NEW different pump assy.

then the frame rail ground needs cleaning or repair for a low resistance ground contact.

also engine goes into closed loop at 160 deg f . so if it never is in open loop the ect or ect wiring is shorted out. low resistance causes closed loop.

poor cold start can be this .. cold start PCM increases injector ON time [MORE FUEL] IN OPEN LOOP .

20GMC00
05-25-2016, 09:33 AM
I did solder in the new connector. I wil look for that ground tonight. Thanks.

20GMC00
05-25-2016, 09:57 AM
It has 270,000 miles. When does the fuel spider usually go bad?

20GMC00
05-25-2016, 07:44 PM
I pulled the upper intake off and the back half of the lower intake is shiny so I guess that answers that question.

20GMC00
05-31-2016, 11:49 AM
Now it starts right when the key is turned and I'm getting P0171 & P0174. Haven't found any vacuum leaks either. About had it with this thing.

20GMC00
05-31-2016, 12:48 PM
Both LTFT are constantly at 25%. I'll be getting some for sale signs today. Thanks for the help.

j cAT
06-02-2016, 08:12 AM
Both LTFT are constantly at 25%. I'll be getting some for sale signs today. Thanks for the help.


the positive 25% fuel trims both banks is a vacuum leak or low fuel pressure.

if the fuel trims drop off on the higher above idle RPM then it is a vacuum leak .

do the smoke test much faster to find the leak.

also do a reset on the PCM so that the fuel trims can re-learn . sometimes people get confused with the fuel trims because of the PCM memory still responding to the past .

Blue Bowtie
06-02-2016, 07:37 PM
If this is a central injector pod (spider) type injection system, verify that the pod seal is installed correctly. You may be able to get a reasonably good look at it from the top, without having to pull the upper intake. There should be the orange quad ring type seal visible all the way around the pod opening in the upper intake.

Don't bother asking me why I know this can be an issue...

20GMC00
06-05-2016, 01:40 PM
When I spay carb cleaner around the intake, I can hear it backfire a little when I spray between the distributed and the intake. I tried the smoke test and there were no visible leak with that.

20GMC00
06-05-2016, 01:41 PM
Damn autocorrect.

j cAT
06-05-2016, 05:03 PM
When I spay carb cleaner around the intake, I can hear it backfire a little when I spray between the distributed and the intake. I tried the smoke test and there were no visible leak with that.

do the smoke test again. pressurize the intake a few PSI.

use condom on the TB air intake bore . inject smoke into the PCV hose to the TB base intake manifold .

20GMC00
06-05-2016, 06:36 PM
What psi do you recommend? Thanks.

j cAT
06-05-2016, 09:00 PM
What psi do you recommend? Thanks.

like 3to 5 psi .put a plug on the exhaust pipe also as the smoke will blow out from there . 3 psi should be good with the exhaust plugged up.

with what you said it does appear to be an intake leak somewhere on the top of the engine.

20GMC00
06-05-2016, 09:15 PM
I'll try both methods again in a little while. Thanks.

20GMC00
06-05-2016, 11:23 PM
I tried the smoke again. Found nothing. Tried carb cleaner again. Didn't find anything there, not even a backfire in the intake like before. Watched the fuel trims and both stayed at 25% and the 02 sensors never peaked at all. I'm lost. Good idea on the condom usage by the way.

20GMC00
06-05-2016, 11:32 PM
These codes came up after I replaced the spider so I screwed up somewhere. Will pull the plenum off tomorrow and see what I find. Also, where the connector for the spider plugs in, how for up should the spider be? Right now the 2 tabs on the connector go down and touch the plenum before they latch. Thanks.

j cAT
06-06-2016, 08:34 AM
These codes came up after I replaced the spider so I screwed up somewhere. Will pull the plenum off tomorrow and see what I find. Also, where the connector for the spider plugs in, how for up should the spider be? Right now the 2 tabs on the connector go down and touch the plenum before they latch. Thanks.

yes the spider install could cause the leak .. injector seals are also a common screw up when replacing .

I would find out where its messed up by smoke test because the carb cleaner does indicate this is leaking..

the wiring connectors touches the manifold does not seem correct.
see if you can get a video on this install.

20GMC00
06-08-2016, 08:40 PM
I just got the plenum off. It was leaking smoke right in front of the distributor. When I got it off, I noticed the gasket was smashed level back there but it stuck down everywhere else. Also there is oil back there. Is that normal? Thanks.

20GMC00
06-10-2016, 11:22 AM
Got a new gasket put on it and it ran like new.....for a couple days. Now I'm getting P0174 by itself. It starts right up and when I put it in gear, it hardly wants to move and is backfiring in the intake. If I put the pedal a ways down it suddenly clears up and takes off like new. How could it be lean on just on bank?

20GMC00
06-10-2016, 12:41 PM
Also the STFT is at 99.2% for both rear O2 sensors. I think I'll just run it the way it is. Tiring of messing with it.

j cAT
06-11-2016, 08:40 AM
Also the STFT is at 99.2% for both rear O2 sensors. I think I'll just run it the way it is. Tiring of messing with it.

if it did run good for several days not the 174 code then the gasket is messed up.

installing an intake manifold use bolts with no heads on them, to drop down on block , then this will prevent the gasket getting messed up .. also follow the torque procedure. could be the intake is distorted.

I do not recommend any silicone sealers . takes too long to cure .

time to smoke test again. runs good OFF idle is an indicator of the vacuum leak.

20GMC00
06-11-2016, 11:34 AM
Went to drive it today and it won't start unless I put the pedal to the floor. Then when it starts it barely has enough power to move. I plugged in the scanner to see what codes there were and it's not even powering the scanner on.

rhandwor
06-11-2016, 11:44 AM
Auto Zone or Advanced Auto has loaner tools. Get a fuel pressure tester and check the fuel pressure. Post the results.

j cAT
06-11-2016, 12:37 PM
Went to drive it today and it won't start unless I put the pedal to the floor. Then when it starts it barely has enough power to move. I plugged in the scanner to see what codes there were and it's not even powering the scanner on.

scanner tool gets power from cigar fuses. see if the power sockets have power check fuses.

20GMC00
06-11-2016, 03:44 PM
Fuel pressure is at 62 psi when the engine is running. Changed the fuse and still only getting P0174. Pickup has no power to move itself and shakes very bad.

20GMC00
06-12-2016, 10:11 PM
I got it home going about 5mph. Couldn't go any faster. Luckily I was only a mile from home.

Blue Bowtie
06-13-2016, 07:24 AM
This now sounds like flooding. Perhaps a leaking injector, regulator, or fuel line in the plenum. Does the fuel pressure drop off quickly once the pump is shut off?

20GMC00
06-13-2016, 09:25 AM
I will check that tonight. I thought it sounded like the timing was off. The hold down on the distributor was tight but the paint mark was way off. Not sure what happened there.

20GMC00
06-15-2016, 07:24 AM
I also took the cap & rotor off and they are toast. They are probably 5 years old. Isn't that a little soon to be shot?

20GMC00
06-15-2016, 08:17 PM
Turning the key on is 65 psi. Turn the key off and the fuel pressure drops to 0 psi in 5 seconds.

Blue Bowtie
06-15-2016, 10:02 PM
That could be a regulator, fuel line, or the pump leaking back. Based upon the reported symptoms it sounds like the pump may not be the prime culprit.

20GMC00
06-16-2016, 08:15 AM
No fuel lines are leaking. The spider assembly and fuel pump are new. What's the best way to find out what's leaking?

20GMC00
06-16-2016, 05:31 PM
With the return line pinched off the pressure still drops right to zero. Would that mean the check valve in the pimp is bad? Thanks.

20GMC00
06-17-2016, 10:15 AM
It was spraying out of the regulator.

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