Hesitating While Driving - Fuel Pump?

[bigcoconut]

99 van with 120,000 miles on it. Just got back from a trip to Kansas City, I live in Houston. Last summer I gave it a tune-up, plugs & wires and filters. Just changed the fuel filter before the trip. On the way up, no problems, smooth as glass. However, on the way home it started hesitating/bucking at highway speeds. Made it to Oklahoma City and stopped at a chevy dealer. The tech drove it and it was hesitating, he had his computer hooked up and it was showing to be running lean, why we don't know. Got back to the shop and hooked a fuel pressure gage and drove it around again, but it stopped hesitating, the pressure was right on the money the whole time. He checked the fuel for water or other contaminants and it was clean.

He said his best guess was the fuel pump, but he said it was a "$600 guess". It was running lean but no codes were recorded by the computer. It started running fine so I took a chance and we filled up and drove the rest of the way with no issues at all, smooth as glass all the way home.

That was a couple of weeks ago, no problems until now, it is hesitating again, just like before. Driving along nothing unusual, and it just starts bucking and then it goes away. I just filled the tank again, but this time it is still hesitating. No "Check Engine" light.

[cjstew4]

Originally Posted by bigcoconut

99 van with 120,000 miles on it. Just got back from a trip to Kansas City, I live in Houston. Last summer I gave it a tune-up, plugs & wires and filters. Just changed the fuel filter before the trip. On the way up, no problems, smooth as glass. However, on the way home it started hesitating/bucking at highway speeds. Made it to Oklahoma City and stopped at a chevy dealer. The tech drove it and it was hesitating, he had his computer hooked up and it was showing to be running lean, why we don't know. Got back to the shop and hooked a fuel pressure gage and drove it around again, but it stopped hesitating, the pressure was right on the money the whole time. He checked the fuel for water or other contaminants and it was clean.

He said his best guess was the fuel pump, but he said it was a "$600 guess". It was running lean but no codes were recorded by the computer. It started running fine so I took a chance and we filled up and drove the rest of the way with no issues at all, smooth as glass all the way home.

That was a couple of weeks ago, no problems until now, it is hesitating again, just like before. Driving along nothing unusual, and it just starts bucking and then it goes away. I just filled the tank again, but this time it is still hesitating. No "Check Engine" light.

If the codes don't point to anything, I would run some gas treatment /injector cleaner through it before looking to the fuel pump.

[rockwood84]

when it hesitated was it smooth highway or bumpy? because your problem could be in the pass-thru connector in the floorboard behind the drivers seat. it the bigger of the two connectors. it has the fuelpump wires, fuel gauge wires going thru it. this connector gets wet from a bad designed seal,the seal is supposed to keep the moisture out but does not.if the connector is green inside and the road is bumpy you could be loosing connection at the pins in the connector.sometimes the fuel gauge will go nuts as it hesitates.i have seen some that you could clean with electrical cleaner then gobb the connector full of diealectric cream. i can't remember how many pins in the coonector seems like it was six or eight . do not scrape on the pins much or they will break off.

[bigcoconut]

It is still having problems, however, now sometime when it hesitiates it will stall. It restarts immendiately, like nothing is wrong.

No bumps or other sudden outside forces acting on the car, just smooth roads and all of the sudden it starts hesitating, the car slows down almost like an engine brake, and then it goes away and I can accelerate back to speed until next time.

I did run some injector cleaner in this last tank but haven't run through it yet, so I don't know if it worked.

Again, no codes are getting stored in the computer, the computer thinks everything is OK as far as what it monitors. The only thing it tells me is that the engine is running lean when it is hesitating, but why is it running lean I don't know.

[bigcoconut]

Would the O2 sensor play a role in this? I figure if the O2 sensor is telling the computer that there is too much fuel in the exhaust then the computer would lean the mixture to compensate, when in reality that just thins the fuel already too much. Should the computer pick this up as a bad O2 sensor or could it be just enough of a voltage fluctuation to fool the computer for a second or two? Again, I'm getting no codes in the computer.

[rockwood84]

Originally Posted by bigcoconut

Would the O2 sensor play a role in this? I figure if the O2 sensor is telling the computer that there is too much fuel in the exhaust then the computer would lean the mixture to compensate, when in reality that just thins the fuel already too much. Should the computer pick this up as a bad O2 sensor or could it be just enough of a voltage fluctuation to fool the computer for a second or two? Again, I'm getting no codes in the computer.

i'm still thinking that the fuelpump is back pressuring. running but not pumping close to 50 pounds pressure. the pressure is probably down to 20-30 pounds and this is not enough to keep the motor running at highway speed,so it slows down .these fuelpumps are bad about this.to find this you have to put a fuel gauge on the fuel line.i put mine on the fueline where the fuel filter goes.and just leave the key on and watch the pressure or you can rig one to fit in like the fuelfilter and put a long hose on it and put the pressure gauge on the dash where you can see it when it acts up.this takes a little more rigging but you can drive it and then see whats going on with the fuel pressure when it acts up. you just have to make a fitting and tubing to fit where the fuelfilter goes and not leak. did you check the pass-thru connector? could be the connector on top of the fuel pump as these will go bad .g.m. used too small gauge of wire when they made them and they get hot and go bad.mine would like yours then it got to bucking and backfiring. but five minutes later it would run 100 mph for as long as you wanted to hold it there. then 3 hours later same bucking. finally rigged up the drivo-gauge and when it would start bucking and backfiring it had a fuel pressure of 20 pounds then go down to 10 pounds back up to 20 pounds.put a new $320.00 fuelpump in and cured it.i used the nipples off of an old fuel filter and put a "t" between them and got 4 foot of hose to put through the window to put the fuelgauge on the end of it.one hint that the connector on top of fuelpump or the passthru connector will give is that the fuel gauge on the dash will act up. if its just a worn out fuelpump the fuel gauge won't act up[go to empty or any other mark ,other than whats actually in the tank]. the reason your getting a lean burn is low fuel pressure.if you get a code to pop it will be for a lean burn and it could be anything associated with a lean burn,tps code. if it backfires or bucks it will pop a code for camsensor ,crankshaft sensor, and tps sensor.but i'm betting on the fuelpump doing the backpressure thing. good luck

[bigcoconut]

Fuel gauge is working just fine, no issues there. The dealer did run a fuel pressure gage taped to the window when we were in OKC, but it never acted up, the pressure was good (per the tech, I couldn't read it from where I was sitting) the whole time.

I'll try to rig a pressure gauge this week and drive it around, I'll let yo know what I find...

[Schrade]

Originally Posted by bigcoconut

I'll try to rig a pressure gauge this week and drive it around, I'll let yo know what I find...

EXCELLENT!!! Diagnostics!!! GOOD WORK!

He said his best guess was the fuel pump, but he said it was a "$600 guess".

This was not a mechanic that said this right? This was a friend who knows little about cars? Because if it was a mechanic, I would say, "I guess I WON'T pay you if it doesn't fix the problem.", and then I'd say, "Never mind, that's not a guess, it's a definite - I WON'T pay."

Do an ohm test of the injectors. Hot, and cold.

Post results.

[pastaben]

sounds like a problem I had on a couple of occasions:
1. kinked throttle cable under hood. some mechanic pushed it out of the way to get at something and left it there, but had to replace it because it was kinked.

2. about 30K miles later, similar symptoms, and replacing the ignition coils did the trick.

[bigcoconut]

OK, I didn't run a pressure gauge on the fuel system like I said. I did a knee jerk reaction, the mechanic in OKC said "maybe" Fuel Pump so that's what I did this weekend along with the fuel filter and guess what, no change )$%&*@#*%^@*( .

So here I am, new fuel pump assy (the whole "kit"). Just dropped the tank, unscrewed the big a$$ black ring, pulled old fuel pump assy out and put the new one in and reverse order everything back together. Pretty easy stuff, the hardest part was making sure I didn't blow myself up with the fumes, thank goodness it was a clear weekend. There was one catch, it came with a new electrical pigtail because the connection at the top of the pump was different. It also had me install a 20A fuse in place of the original 15A at the fuse block (#32 for the fuel pump).

OK, so I'm at square one again, the engine hesitates/slows down (it isn't stalling) at all speeds, just taking off and cruising along, the van slows down, jerks around a bit, and then it comes back and I can accelerate again. BTW all this bagan after my 100,000 mile tune-up, new plugs (Bosch), new wires (AC/Delco), filters (air and oil), PCV, fluids check, and new fuel pump assy, etc...

My assumption now is the fuel delivery system is OK up to the injectors, there is something telling the computer to lean the fuel mixture or something is leaning the fuel mixture on its own, and the computer isn't catching it because there are still NO CODES. BTW, ran a tank of "injector cleaner" with no change, just filled up second tank with "injector cleaner" for a second dose.

[rockwood84]

at 120,000 miles the fuelpump was about to the end of its life anyway .some of these mexican made fuelpumps [thanks to nafta] don't make it past 90,000 miles.did you check the connector in the floor board? one other thing ohm the tps [ the numbers should change as you move the notch back and forth.and see if it is in the notch on the throttle body should be centered with notch in the groove on the tps.if its not centered it will throw off the computer.or the tps could be cracked does it act like a miss or series of misses? did you get the map sensor hoses and wires back on? since it started after the 100,000 mile tune -up check that all the connectors are on and every hose is hooked up. its easy to knock one loose or an old hose to break.pull the vacumn line off the fuel regulator and see if gas comes out.

[bigcoconut]

All underbody connections are good, checked them and used dielectric grease on all.

It feels like a series of misses, not just one but multiple cyclinders.

Triple checked the MAP sensor hoses, had them reversed after tune-up, ran like crud for awhile and through a code (don't remember what but it pointed me right to the MAP sensor). All's good now, as far as that is concerned.

I can check the TPS (throttle position sensor) tonight, thats relatively simple to do I think, I'll post results. While there I'll triple check all my connections again.

[bigcoconut]

Give me a hint as to where the fuel regulator is, I've got a GM maintenance manual on this thing, reads like stereo intructions (need a class on those things), but it's good at procedures once I nail a part down.

[merc81]

Here is a photo of the fuel regulator with the vacuum line removed.
John

[rockwood84]

could be a coil getting hot but it would keep missing until you shut the motor off then it would run fine until it got hot again.in other words it wouldn't run fine then miss then run fine.and the 7x cranksensor on the backside of the motor could make it miss but should pop a code .as any miss fire should have popped a code.if the 24x cranksensor behind the harmonic balancer was acting up the tach would get screwy then eventually it wouldn't even crank up.