99 tahoe misfire

[1987ss]

i have a 99 tahoe other day i started it and it ran rough so drove it home check engine light flashes at about 45 mph when i got home the cat was glowing so had it scaned n cyl 3 n 5 were misfire .. cut off cat same thing stil misfire. changed plugs cap rotor wires had cat welded on again. and put in a used spyder. now it only misfire on 5 n occasional 7. has compression 176 using a snap on scanner n compression tester. please help what are odds on another bad spyder? any feed back helps!

[1987ss]

please help

[MT-2500]

Originally Posted by 1987ss

please help

What engine?
Any other codes?
Fuel pressure?
Good Ac Delco parts?
Good hot spark to all plugs?
What is the camshaft retard setting reading?
How many misfire per hr. on no 5?
Any mods or add on or recent repair work?
What is the mileage on it?

[1987ss]

Originally Posted by MT-2500

What engine?
Any other codes?
Fuel pressure?
Good Ac Delco parts?
Good hot spark to all plugs?
What is the camshaft retard setting reading?
How many misfire per hr. on no 5?
Any mods or add on or recent repair work?
What is the mileage on it?

no other codes no crank cam correlation snap on says fine to drive its a 350 200k well maintain spark is fine all parts ac delco fuel pressure engine off key on 61 psi n holds for afew sec then leaks down ... engine on 53 psi

[MT-2500]

Originally Posted by 1987ss

no other codes no crank cam correlation snap on says fine to drive its a 350 200k well maintain spark is fine all parts ac delco fuel pressure engine off key on 61 psi n holds for afew sec then leaks down ... engine on 53 psi

What is camshaft retard reading?
We need a full fuel pressure check to rule out low fuel 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.

[j cAT]

Originally Posted by 1987ss

no other codes no crank cam correlation snap on says fine to drive its a 350 200k well maintain spark is fine all parts ac delco fuel pressure engine off key on 61 psi n holds for afew sec then leaks down ... engine on 53 psi

your engine with its mileage has many , that needed a new distributor ..the bearing wear causes the rotor to move around too much...

[1987ss]

Originally Posted by MT-2500

What is camshaft retard reading?
We need a full fuel pressure check to rule out low fuel 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.

fuel preasure checked out.. brought ot on it was a bad cap! thanks 4 post back.. now after truck warms up it hesitates have to baby the pedal give it gas gradualy..if u turn it off n restart it it runs fine for a while.

[j cAT]

Originally Posted by 1987ss

fuel preasure checked out.. brought ot on it was a bad cap! thanks 4 post back.. now after truck warms up it hesitates have to baby the pedal give it gas gradualy..if u turn it off n restart it it runs fine for a while.

so the distributor cap was bad ? if so how/why ?

this engine at your mileage should need a new distributor ..how did the rotor feel on movement ?

[777stickman]

Took me 10 minutes of reading before I finally saw that the motor is a 350 (5.7).

You say that the fuel pressure is OK, but it's not, at 53 psi, with motor running. Re-read MT's reply on fuel pressure.

As j cAT says, distributor problems are an issue with the older cars. So, to be specific on checking this dist, pull the cap and do these checks with the rotor in place:

1. up & down play (1/16") is good and ok.
2. Any rotational play is not good! (Hold down pressure on the rotor and gently try to rotate in both directions).
3. Any side to side play is not good!

Good luck to you.

[1987ss]

rotor felt good.. but truck still has a occurring issue only in summer months.. it will hesitate when running for about a hour u can give it gas and it goes flat but shut it off starts and runs normal for a while... oh yeah when it acts up u can give it gas gradually ond keeep it goin.. think fuel pump may be it after compairin to MT specs