Stumbling

[thomnjohn]

About a month or so ago my 97 Blazer started stumbling upon acceleration. This only happens once the engine is warm. It gets worse the farther I try to drive it after it starts stumbling. If I turn off the ignition for about 2 seconds and start it back up, it seems to reset something and runs fine for a few more minutes. I changed the fuel pump about a month ago as well. Coincidence??? Is there a way I can check the fuel pump to see if I have a faulty one?

The only reason I changed the pump in the first place was because my fuel gauge was not working. I bought a fuel sensor off ebay, but I broke one of the plastic nipples off the original pump while removing the tank. (Lesson learned the hard way)

It almost feels like my 92 S10 did when the Cat got clogged. Could this be the culprit?

Any other suggestions or things to check would be greatly appreciated.

[Yogi]

To check the fuel pump you will need a pressure gauge - about $40 at AutoZone. Pressure should be 55-60 psi with key on engine off.

Clogged cat is unlikely because if this was the problem It would not run any better when you shut it off for 2 sec. and restart it.

What is the condition of plugs, wires, cap & rotor? How long since you changed the fuel filt.?

[thomnjohn]

I've only owned the truck for 3 months now. New plugs, wires, cap and rotor are on the way. A new fuel filter certainly won't hurt my cause. These are things that need to be done regardless. I guess I'll go get a pressure gauge this weekend and check the pressure. I'm not totally convinced that replacing these items will correct my issue though. Why would it run fine after I kill the engine and refire immediately? I don't have to let the truck cool down. It runs fine for a little while immediately after I refire.

[Yogi]

It could be something weird like an IAT sensor, but it is best to start with the basics and go from there.

When you go to get the fuel pump at the parts store, have them scan the pcm for any stored codes.

[thomnjohn]

I went to Auto Zone to get the code read, but there were no codes stored. The Service Engine Soon light came on Friday on my way home, but did not come on when I started the truck Yesterday or today. I haven't checked the fuel pressure yet, but why would there be no codes stored if the Service Engine light came on???

[Yogi]

There are hard codes and there are soft codes. A hard code sets the light and records a code in memory that remains through a certain number of engine start/stop cycles. A soft code sets the light, and sets a code in memory that remains as long as the fault is present, and the light stays on. When the light goes out, a soft code will be erased from memory. So, for soft codes, you have to scan it while the light is on in order to catch it.

[PDW]

this is happening to me too, and its driving me freaking nuts. I have a 99 extreme A/T.. I run fine for a while then it stutters and stalls. Let it sit for a while and itll run again for a few minutes.. . I changed the gas tank recently to fix a prior issue. To try to fix this one I changed the fuel pump, plugs, wires, coilpack, pcv, air filter, fuel filter and while we were at it we did an oil change just for the heck of it. And it ran fine for a month...until today..Back to crap.... Tomorrow we are doing a backpressure test to see if its the cat.. thats what everyone is telling me it is so I figured I tried everything else may as well do this too...
It is also throwing a bunch of 02 sensor codes the guy said last time but didnt tell me what they were. Cel comes on once in a while..

[thomnjohn]

Alright, I had to have the truck towed home Wednesday. It died right in the middle of traffic. It started right up and I was able to limp it into a parking lot. I gotta figure out what's wrong now.

I checked my fuel pressure at the fuel rail and it's reading 59 PSI with the ignition ON. When I start it up it drops to about 52 - 55 PSI. (Haynes manual says it should drop 3 - 10 PSI when running) I ran the truck until it started stumbling, then I pressed on the throttle at the throttle body so I could keep an eye on the gauge while making the engine stumble and try to die. The pressure stayed at a constant 52 PSI. This would lead me to believe that it's not the pump nor the filter. I bought a filter for the heck of it and will put it on tomorrow.

If clogged injectors were my problem, shouldn't the pressure at the fuel rail drop when it's stumbling?

I'm going to replace the plugs, wires, cap, and rotor tomorrow. After reading the post above and many other posts on this issue, I'm not sure a tune-up is going to do much good. It almost feels like a sensor, but which one. Any other suggestions or things to check?

[Yogi]

Quote: Originally Posted by thomnjohn

Alright, I had to have the truck towed home Wednesday. It died right in the middle of traffic. It started right up and I was able to limp it into a parking lot. I gotta figure out what's wrong now.

I checked my fuel pressure at the fuel rail and it's reading 59 PSI with the ignition ON. When I start it up it drops to about 52 - 55 PSI. (Haynes manual says it should drop 3 - 10 PSI when running) I ran the truck until it started stumbling, then I pressed on the throttle at the throttle body so I could keep an eye on the gauge while making the engine stumble and try to die. The pressure stayed at a constant 52 PSI. This would lead me to believe that it's not the pump nor the filter. I bought a filter for the heck of it and will put it on tomorrow.

If clogged injectors were my problem, shouldn't the pressure at the fuel rail drop when it's stumbling?

I'm going to replace the plugs, wires, cap, and rotor tomorrow. After reading the post above and many other posts on this issue, I'm not sure a tune-up is going to do much good. It almost feels like a sensor, but which one. Any other suggestions or things to check?

Fuel pressures look good. How was it running when you tested? It would be helpful to get fuel pressure when it is running bad.

If clogged injectors were the problem, fuel press. would be normal to high - not low. Get a can of starting fluid. Next time that it stalls, try spraying starting fluid into the intake. If it starts on starting fluid, that would indicate an injector problem.

Test your ignition coil. Click here for instructions.

Have you tuned it up yet? Results?

Next time that it acts up, try tapping on the EGR valve. Let us know what happens.

[thomnjohn]

It idles fine. It only stumbles under throttle. I throttled it up at the throttle body until it started stumbling and kept an eye on the gauge. It never dropped below 52 PSI. It didn't spike either.

I haven't yet tuned it up. I plan to do that one evening this week.

The truck runs and idles fine. It won't start stumbling until I actually drive it. It will sit and idle all day long. I even revved it up and ran the A/C for several minutes and couldn't get it stumble. Just as I took it out for a drive, it started stumbling.

I'll check the ignition module and tap the EGR valve the next time I work on it.

Thanks for the ideas. I'll let you know what I find.

[thomnjohn]

Still haven't tuned it up yet, but I did check the Ignition Coil and it checked out fine. After reading the SCFI to MPI thread, I think the spider injector may be my problem. Funds are a bit low right now and I've been riding the bike to work, so I'll get it tuned up eventually and go from there.

[Yogi]

Quote: Originally Posted by thomnjohn

Still haven't tuned it up yet, but I did check the Ignition Coil and it checked out fine. After reading the SCFI to MPI thread, I think the spider injector may be my problem. Funds are a bit low right now and I've been riding the bike to work, so I'll get it tuned up eventually and go from there.

I was going to suggest the FI, but I thought it best to eliminate the ignition and the EGR first.

Hang in there!

[PDW]

went in to the dealer and had it diagnosed as a fuel pump...AGAIN... so we changed it AGAIN. The one we initially put in was "not oem and spraying at 50 when it should be 60". But it (the part) was under warranty so it didnt cost anything.
Seems to have fixed the problem, but I guess only time will tell..

[thomnjohn]

I ordered the SCPI to MFI conversion kit Saturday and bought new plugs, wires, cap and rotor. I plan on doing the tune up when the new spider gets here. Hopefully this finally solves my issue.

[Yogi]

Good luck with the tune-up and conversion!

[thomnjohn]

Still waiting for the conversion kit to ship from gmpartsdirect. I went ahead and tuned it up today. I can tell a noticeable difference in idle and accelaration, but it still stumbles once it gets warm. Hopefully my kit ships this week.

[Yogi]

Yeah, let's hope!

[thomnjohn]

Well my kit came today and I installed it. It runs a bit longer now, but still with stumbling. It's acting the same way it did when it first started. It gets worse the longer I drive and seems to reset when the ingition is turned off and restarted.

I feel like I'm chasing my tail. I really want to fix it myself, but I'm losing hope and patience quickly.

I still think it kinda feels like my S10 did when the Cat got clogged. I'm wondering if when it warms up the honeycomb is expanding and not allowing the exhaust to exit as freely as it should. I plan on honing out the Cat tomorrow.

Any other suggestions???

[Yogi]

If hollowing out the cat doesn't do, then I'm out of suggestions.

Still no check engine light?

[Bill Wheats]

Quote: Originally Posted by thomnjohn

...I still think it kinda feels like my S10 did when the Cat got clogged. I'm wondering if when it warms up the honeycomb is expanding and not allowing the exhaust to exit as freely as it should. I plan on honing out the Cat tomorrow.

Any other suggestions???

It doesn't sound like your cat, if restarting it solves the problem for a while.

When you restart your engine, the computer goes back to open loop and doesn't use info from several sensors. It also won't run your EGR valve until it goes into closed loop.
My guess is a wacked out O2 sensor or stick egr.

I'd recommend conecting a vacuum gauge and running some tests when the engine is cold, then again when it is warm and messing up.
Running some Seafoam through it might not be a bad idea.

[thomnjohn]

Still no Check Engine Light.

I ran Seafoam through it a couple of times before the conversion with no luck. I took the EGR valve off and cleaned it out and the plunger didn't seem to be sticky at all. If it were an O2 sensor wouldn't it trigger the Check Engine Light?

Here's a list of things that have been replaced so far.

Fuel pump
Fuel filter
Plugs, wires, cap, and rotor
Camshaft position sensor
MFI injector conversion

[thomnjohn]

I cleaned the EGR again and hollowed out the cat a bit. I didn't completely gut it and I made sure to get all the loose pieces out.

Still no improvement.

Tonight I started it up and let it idle for about 30 mins then I drove it for about 20 mins. It ran fine. I then parked it and let it sit idling for about 20 mins. When I went back out to take it for another drive, it would stumble when I gave it throttle.

I guess I'm going to have to take it to someone who knows what they're doing, because I sure don't have a clue.

[Yogi]

If you do, be sure to let us know what they find!

[thomnjohn]

Mechanic says it's a bad fuel pump. He hooked his gauge up and drove it until it started stumbling and the pressure dropped to 20 psi then down to 0. Only time will tell if this is truly my problem.

[Yogi]

Sounds like the "new" fuel pump is bad!

[awbarron]

Any updates... I've had the same problem and replaced most of the same parts so I'm looking for as much help as possible.

[thomnjohn]

Alright, the truck is now fixed. It turns out when I wired up the new pigtail for the new pump, I didn't have one of the grounds in the butt connector good enough before I crimped it. My mechanic said the pump was essentially shorting out and after a period of time (usually about 25 mins) the pump would start lagging. O'Reillys replaced it without question.

Two things I learned from this whole experience.

1. Take your time when wiring up the new pigtail. I've spent a lot of time and money chasing a problem that I caused. I've wired up plenty of audio equipment with no problems. I guess I was in a hurry or just wanted to get out from under the truck.

2. When I first checked the fuel pressure, I should have ran it up through the hood and taped it to the windshield. That way I could've monitored the pressure while I was driving it and it was acting up. (Yogi, you suggested this in post #9. I should've listened.) This is how my mechanic determined it was the pump.


And as if my luck wasn't bad enough, my windshield got busted while at his house. He had my truck parked by the edge of the woods by the shop and a limb fell on it and busted it. Didn't do any other damage and he has replaced the windshield.

[Yogi]

Glad that you got it worked out!!

[1droptine]

[quote=thomnjohn;5408170]Alright, the truck is now fixed. It turns out when I wired up the new pigtail for the new pump, I didn't have one of the grounds in the butt connector good enough before I crimped it. My mechanic said the pump was essentially shorting out and after a period of time (usually about 25 mins) the pump would start lagging. O'Reillys replaced it without question.

Pulled bed off 2000 4.3 yesterday because second pump not pushing any pressure now. Cut off butt connectors to pigtail on airtex pump and soldered connections to ensure good contact, still nothing. I then added a temp ground wire at solder joint to frame and pump turned on and shows 50lbs pressure then droppes off but would not start. Thinking I may have damaged harness when dropped tank for first pump replacement since both pumps did same thing. Anybody know where I could buy new harness and if the pump may now be damaged and need replaced? Thanks

[Yogi]

Do you mean like this?
http://198.208.187.182/servlet/com.e...partnbr=PT2054
Rock Auto has em.

Edit: I don't know why the pic won't post. Just click on the Rock Auto link and then click on "More info"

[1droptine]

Actually looking to find the main harness that supplies power from fuse block to pump. Guessing its a dealer part because I cant find it online. Want to replace both pump and harness.

[Yogi]

Rock Auto recently advertised that it now has wiring harnesses for sale. Evidently they don't show up online. You might want to call them before you get hijacked by a dealer. Current Performance is a sponsor for the Forum that makes complete engine harnesses too, so I'm sure that they could also provide it.