Computer question 1999

[YooperJ]

This is related to my poor running problem in my 1999 Bravada. It runs rough and backfires upon acceleration for approx 45 seconds and then is fine. It does this on any startup, any engine temp and any ambient temp. I'm thinking there must be a timer in the computer that is making it run (lean?) bad for 45 sec. I can't think of anything else that would do this for a specific time period under different operating parameters. Anybody hear of anything like this or am I missing something obvious. No trouble codes, ECT, IAT, MAF, MAP all seem to be within specs. Converter has no back pressure buildup, under .5 psi. Fuel pump is within specs.

[MT-2500]

Originally Posted by YooperJ

This is related to my poor running problem in my 1999 Bravada. It runs rough and backfires upon acceleration for approx 45 seconds and then is fine. It does this on any startup, any engine temp and any ambient temp. I'm thinking there must be a timer in the computer that is making it run (lean?) bad for 45 sec. I can't think of anything else that would do this for a specific time period under different operating parameters. Anybody hear of anything like this or am I missing something obvious. No trouble codes, ECT, IAT, MAF, MAP all seem to be within specs. Converter has no back pressure buildup, under .5 psi. Fuel pump is within specs.

Always check secandary ign first.

Plugs wirers cap and rotor?
HOW MANY MILES ON TUNE UP?
GOOD ac PLUGS?
What is camshaft retard reading?
What is the fuel presure readings?

[YooperJ]

There is about 20K since tune-up. AC plugs used and good. Fuel pump good.
Cap and Rotor look good. Didn't get a chance to check Cam retard yet. Runs bad for only about 45 sec and then it is fine.

[MT-2500]

Originally Posted by YooperJ

There is about 20K since tune-up. AC plugs used and good. Fuel pump good.
Cap and Rotor look good. Didn't get a chance to check Cam retard yet. Runs bad for only about 45 sec and then it is fine.

Can not always tell by looking at cap is it is good.
What is fuel pump pressure readings?
Any codes?

[YooperJ]

No codes. Checked Fuel pump pressure again, Key on-Engine off = 62 psi then lowers to about 56 or so. Engine idling = 55 psi and when I accelerate it pressure jumps to 62 and then back down. At 1000, 1500, 2000 & 2500 RPM the pressure remains at 53 psi.

Vacuum is 19 in at idle and at 1500-2000 RPM it goes to 20 in. When I accelerate it goes to 23 in and then back to 19 in.

Wouldn't a bad cap always be bad and not get better after 45 sec?

[MT-2500]

Originally Posted by YooperJ

No codes. Checked Fuel pump pressure again, Key on-Engine off = 62 psi then lowers to about 56 or so. Engine idling = 55 psi and when I accelerate it pressure jumps to 62 and then back down. At 1000, 1500, 2000 & 2500 RPM the pressure remains at 53 psi.

Vacuum is 19 in at idle and at 1500-2000 RPM it goes to 20 in. When I accelerate it goes to 23 in and then back to 19 in.

Wouldn't a bad cap always be bad and not get better after 45 sec?

Fuel pressure is low.
Can the fuel pump reach 95-90 lbs full pressure?
It should have 62-65 lbs running pressure.

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 or hot and fuel pressure testing guide lines.
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.

[b1lk1]

I just had to replace a 3 month old cap on a vortec 5.0L engine that went bad. Cannot tell by looking at them at all.