engines misses @ 65-75 mph

[jerryblazer]

I have a 1997 Chevy Blazer S-10 with a V6 Vortex engine. When I go on the highway between 65-75 mph down a long grade, the engine starts to miss, and then runs poorly until I turn it off and restart it. I have already replaced plugs, wires, cap, rotor, coil, module, fuel pump,fuel injectors, fuel filter, and air filter. It runs very well in city driving and on flat highway driving, but whenever I go down a hill, it starts to miss and runs bad until I stop and turn it off again. No engine codes come up. Any suggestions?

[MT-2500]

Welcome to AF.
Strange it usually misfires going up a hill.
Is down hill only time it misses?
Might pay to get it on a engine capable scanner and read out all engine running data and sensors.
Also there may be codes in the scanner even if no check engine light.

[jerryblazer]

Lately it has been really bad. When I coast along at about 65-70 mph, with my foot barely on the accelerator, it misses so bad, I wonder if it is going to make it. I've already spent almost $2000 on parts. I'm going to repalce the fuel pressure regulator tomorrow. Any other suggestions?

[MT-2500]

Before throwing another 2 grand at it do some proper testing.
Get it on a engine capable scanner and see what is going on.
Does check engine light blink or flash when missing?
Post back any codes.

[jerryblazer]

The engine coded only one time-misfire on all cylinders. I've been told that this is a generic code that appears when the problem is outside the ignition system. That leads me to believe it is probably a fuel problem. I also looked in the manual at the library. Nothing seemed to be on point. However, most problems with an engine misfire upon acceleration or with light throttle, the manual suggested a vacuum problem. The fuel pressure regulator seems to be my last effort. Any other suggestions?

[MT-2500]

You need A full engine capable scanner.
Not just a code reader.
Also check the camshaft retard readings on a engine capable scanner.
If off over 3-4 degrees it can give a missfire.
I would also run a full fuel pressure test to.
Remember proper testing before throwing more parts at it is you best friend.

[jerryblazer]

I brought the truck to 3 different garages for testing, including a GM garage. They gave me a read out of the engine diagnostics. None of them could find anything wrong. The last one I brought it to, I suggested they check the fuel pressure. They told me it was under pressure and that I replace the fuel pump. They also told me that it might be dirty fuel injectors. That's why I had them replaced. Do you think it could be the fuel pressure regulator, or do you believe it is something else?

[jerryblazer]

I know it sounds silly, but after they ran the fuel pressure test, the truck ran really good. But after a few days, it seemed to run poorly again.

[MT-2500]

Miss when you let up on it does not sound like a fuel problem unless the fuel pressure regulator is overfueling
Proper testing.
You need a engine capable scanner read out when it is doing it.
A good scanner will set a code past pressent or history and will readout missfires.
What is the fuel trims reading and what is the camshaft retard readings and watch the tps sensor for drop outs.
Ask them for the fuel pressure read out and post it back to us.

On the fuel pressure run the full test and post back all readings.

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.