99 gmc jimmy starting problem


theyard1
11-03-2007, 01:17 PM
truck will crank but not start all the time some days will start on first crank other days will have to crank it on and off ten minutes then it will start checked fuel pre sure it is good 60 psi but does not hold truck runs fine once started any help would be greatful thank you john:banghead: :banghead: :banghead:

MT-2500
11-03-2007, 03:34 PM
Welcome to AF.
You need 65/66 lbs on a cold start.
Even a few lbs low will not let the injectors squirt on a cold start.

Check the fuel pressure when it does not want to start.
The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

old_master
11-03-2007, 05:19 PM
...fuel pressure it is good 60 psi but does not hold ...

After the fuel pump shuts off, fuel pressure must remain above 55psi for at least 3 to 5 minutes, (with the engine cold). If it drops below 55psi there is a leak. Check for and repair external leaks if/as necessary. If there are no external leaks, the fuel pressure line must be shut off between the pump and the fuel pressure service port with an inline valve after the pump builds pressure. If fuel pressure holds with the valve shut off, the fuel pump check valve is leaking. If fuel pressure still drops, the leak is under the upper intake plenum. Most likely the fuel pressure regulator, lines, or an injector.

theyard1
11-12-2007, 11:46 AM
the fuel presure was around 62-63 psi but leaks down very fast when pump shuts off. what location is best to pinch off the return line

MT-2500
11-12-2007, 12:11 PM
What is cold start cranking pressure?
The full fuel pressure test would help.
Engine cranking cold and engine running and the engine running on the road and the full pump preassure readings.

There is a rubber line between frame and back of engine.
Some pinch it there for a test.
But Better to plug it for a test.

old_master
11-12-2007, 05:06 PM
the fuel presure was around 62-63 psi but leaks down very fast when pump shuts off. what location is best to pinch off the return line

When restricting the fuel return line, do not allow the fuel pressure to rise above 75psi. Excessive pressure will damage the fuel pressure regulator. Restricting the return line will not tell you what's causing the leakdown. To determine what is causing the leakdown, the fuel pressure line must be shut off somewhere between the fuel pump and the pressure test port while the fuel system is pressurized. If fuel pressure holds, the fuel pump check valve is leaking. If pressure still drops, the fuel pressure regulator, the fuel lines under the plenum, or the injector are causing the leakdown.

theyard1
11-13-2007, 10:46 AM
cold cranking presure is 60-63 psi plug return line 90 psi key off drops down to 61 psi and held after i unplug the return and hooked back up the presure held at 61 psi with key off started presure was 58-60 psi drove 45 mph presure 60-61 psi shut off still leaked down but much slower then before

MT-2500
11-13-2007, 11:44 AM
Pump pressure is good.
But running pressure is a little low.
On road you should see 62-65 lbs.
But
Ok on the days it will not start is the time you need to check it.
even a few lbs low will give a no start cold.
You need around 64-65 lbs cold to squirt the injectors.
When no start also check for good spark to more than spark plug.
And for injector pulse.

When no start if it is a fuel problem squirt a little carb cleaner into intake.
If it starts on carb cleaner than you have a fuel problem.
Either low pressure or injector pulse.

theyard1
11-19-2007, 02:58 PM
ok truck did not want to start today tryed spaying carb cleaner also starting fluid would not start spark is good have no way of testing injector pulse. when i go to start it turns over like its trying to fire and then blows white smoke out of the throttle body some times:banghead:

MT-2500
11-19-2007, 06:42 PM
Has it got good spark at the spark plugs?

Good AC delco plugs?
Pull some spark plugs and see they are wet on the end?

skyzend
11-20-2007, 01:02 PM
This intermittent problem reminds me of a problem I had on my 99 Blazer.
It would not start, with intermittant attempts at starting I could get it to go after about 10 minutes.
This progressively got worse until it just would not start at all.
Does you security light flash after the first non start attempt.

This 99 Jimmy likely has a Passlock theft control system which will disable the injectors for 10 minutes after a bad code is accepted when turning the key.
I do not know how the actual ignition switch can intermittantly go bad, but it can and will.

If your security light flashes and remains on, then it is in the 10 minute waiting mode. No starts will be allowed until the Security light goes out.
It will go out after exactly 10 minutes from the first key attempt.
If this is the case ... replace the ignition switch.

As a side-note, much has been written about attempting to bypass this system. I have yet to see a sucessfull story about this ... I would just replace the switch and be done with the problem.

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