Stumbling at highway speeds

[pikappkaz]

Hi all,

Just replaced the CSFI with the MFI spider, and I was hoping to get rid of the stumbling that I had while driving on the highway mostly. I usually get up to around 70mph or so, and then have to get off the gas because the engine is misfiring.

Well the MFI didn't help it, but it still needed to be swapped out. I've checked the fuel pressure at the rail, and its 56psi key on engine off, and 51 engine running at idle. I'm pretty sure that this is too low, it should be above 60 per the manuals. Fuel filter was swapped out with no results.

Is this a fuel pump issue? I'm tired of throwing new parts at this thing, but going down the line, it's looking like the pump isn't giving enough pressure. Any thoughts on this?

Thanks,

Tom

[j cAT]

Originally Posted by pikappkaz

Hi all,

Just replaced the CSFI with the MFI spider, and I was hoping to get rid of the stumbling that I had while driving on the highway mostly. I usually get up to around 70mph or so, and then have to get off the gas because the engine is misfiring.

Well the MFI didn't help it, but it still needed to be swapped out. I've checked the fuel pressure at the rail, and its 56psi key on engine off, and 51 engine running at idle. I'm pretty sure that this is too low, it should be above 60 per the manuals. Fuel filter was swapped out with no results.

Is this a fuel pump issue? I'm tired of throwing new parts at this thing, but going down the line, it's looking like the pump isn't giving enough pressure. Any thoughts on this?

Thanks,

Tom

give us the vehicle mileage engine etc,,,,any maintenence done lately,,,,?

I would keep the fuel gauge on it and with a long hose observe pressures when driving ...53-62 psi is for 99-on up...

remove the vac line from fuel regulator and see if the pressure rises,,also that there is no fuel in the vac line...as this would indicate a hole in the regulator diaphram...

[pikappkaz]

Yeah, sorry about the lack of details, here it goes:

1999 2dr 4wd 5.7l
160,00k

Did the intake manifold 2 years ago, along with plugs, wires and dist. Just finished up the MFI spider last weekend, new plugs went in at that time as well. Fuel filter went in last night, no change in fuel pressure with that swap.

So 53-62 is normal for the pre-2000 body style? Everything I have read says 60-68 KOEO.

I'll give the regulator test a shot today.

[j cAT]

Originally Posted by pikappkaz

Yeah, sorry about the lack of details, here it goes:

1999 2dr 4wd 5.7l
160,00k

Did the intake manifold 2 years ago, along with plugs, wires and dist. Just finished up the MFI spider last weekend, new plugs went in at that time as well. Fuel filter went in last night, no change in fuel pressure with that swap.

So 53-62 is normal for the pre-2000 body style? Everything I have read says 60-68 KOEO.

I'll give the regulator test a shot today.

the pressures I gave are for the 99-up 5.3L....not sure with the 5.7L

[777stickman]

KOEO pressure should be 60-66 psi. May take a couple of cycles of the KOEO to get the readings. Pressure should hold at 55-60 psi after the pump stops.

These figures are out of my '98 C/K service manual for an Old Body Style 5.7.

I would suspect the regulator or the pump assembly.

MT-2500 has an excellent guide for fuel pressure troubles.

If it turns out to be the pump, make sure you get an AC-Delco/Delphi pump that's matched to your VIN.

[Jeremy-WI]

Tell more about the stumble. Is it only at hiway speeds with light pressure on the gas pedal and disappears when you give it more gas?

Fuel pressure should be 60-66 psi KOEO and in the area of 55psi at idle and should jump above 60 with a quick blip of the throttle when running

[pikappkaz]

Well, I went ahead and ordered the Delphi fuel pump, should be here to install this weekend, I'll post with the results.

As for the stumble, the scenario when I can make it happen everytime is getting on the highway, giving a lot of gas on an on-ramp. I will accelerate to about 70, and then the stumble starts to occur. I usually have to get off the gas until I can feel the engine 'catch-up' which will drop the speed down to about 60, then its good to go. Well, good to go until the next time I have to accelerate again, like up a decent hill. You can get it to go away with a hard acceleration during the stumble.

Pressures looked off, so I went with the fuel pump, the spider and regulator were replaced last weekend, so this is the next step.

Any tricks to changing out the fuel pump, or is it pretty straight forward from dropping the tank? Am I going to need a fuel line disconnect tool, or will my ford one work?

[pikappkaz]

UPDATE

I swapped out the fuel pump this weekend, and I am still getting a low reading on the fuel pressure, same exact readings: 56psi KOEO and 51psi engine running. Same problem of stumbling exists on the highway. Could my pressure gauge just be reading low, I haven't been able to try anything different.

What is the next step of troubleshooting? I am lost and very frustrated in throwing money around at this point. The fuel pressure regulator was replaced along with the MFI injection two weeks ago, and the same problem existed pre and post install.

I am going to try driving it with the pressure gauge attached to see if there is any smoking gun there.

Replaced in the last month:

Fuel: MFI injection spider, fuel pump, fuel filter.
Electrical: Plugs, wires, cap, rotor. Everything is AC/Delco or Delphi OEM parts.

Is there another part of the fuel system that I am missing, I have never trouble shot vacuum lines or anything along that part. I am going to do some research and learn about that, but any pointers would be greatly appreciated.

Could it be the fuel pump relay? Even though there is never a problem starting it, could this cause a low voltage on the lines to the pump, causing degraded performance? That's just a thought, not based on anything.

My wrenches can only handle so much throwing.

thanks,

Tom

[777stickman]

If you have a buddy with the same motor you could check the pressure gauge on that.

Wiring, good ground and connections, at the pump area.

Relay could be an issue if you're not getting full voltage at the pump connector. Maybe pull it out and re-seat it or replace it.

The only other thing for low pressure is a clogged fuel line?? I don't have a clue what would cause that.

[j cAT]

Originally Posted by pikappkaz

UPDATE

I swapped out the fuel pump this weekend, and I am still getting a low reading on the fuel pressure, same exact readings: 56psi KOEO and 51psi engine running. Same problem of stumbling exists on the highway. Could my pressure gauge just be reading low, I haven't been able to try anything different.

What is the next step of troubleshooting? I am lost and very frustrated in throwing money around at this point. The fuel pressure regulator was replaced along with the MFI injection two weeks ago, and the same problem existed pre and post install.

I am going to try driving it with the pressure gauge attached to see if there is any smoking gun there.

Replaced in the last month:

Fuel: MFI injection spider, fuel pump, fuel filter.
Electrical: Plugs, wires, cap, rotor. Everything is AC/Delco or Delphi OEM parts.

Is there another part of the fuel system that I am missing, I have never trouble shot vacuum lines or anything along that part. I am going to do some research and learn about that, but any pointers would be greatly appreciated.

Could it be the fuel pump relay? Even though there is never a problem starting it, could this cause a low voltage on the lines to the pump, causing degraded performance? That's just a thought, not based on anything.

My wrenches can only handle so much throwing.

thanks,

Tom

so you installed a new delphi pump,, and the pressures have not changed,,,this is odd...I would expect some difference between pumps.

the voltage to the fuel pump need be checked..also the fuel relay ,, and the fuel pump grounds , need to be examined for proper operation/condition...the ground is at the frame rail next to tank..

try another guage...

you did replace the distributor 2 years ago???? do you think its got a bad bearing..??

[pikappkaz]

OK, what's the best way to get at the fuel pump wiring? Do I need to drop the tank down again, or get some long skinny arms?

New pump relay didn't do anything, best I can tell all of the voltages at the relay look good, all above 12V. I will check out the ground to see if I can access anything.

Distributor was NOT replaced, it was just removed when I did the manifold gasket a few years ago.

Thanks all. Time to make friends with the multimeter.

[pikappkaz]

OK, I have checked the voltage at the pump, 3 different times.

Before replacing any grounds or wires: 11.75V
After replacing the fuse and fuel relay: 11.75V
After rewiring the ground from connector to rail: 11.75V

Voltage at the fuse is 11.96V, so I don't think that slight drop is going to affect it, does it?

I'm still pushing 56psi pressure KOEO, I cannot get this value to rise.

Any thoughts, is this voltage the issue?

[777stickman]

I would say the 11.75v at the pump is fine KOEO. Need to check v's with the engine running. Should be close to 14v. Really need to check your press gauge on another vehicle or try a different one on yours, just to be sure.

Other things to consider are Base Timing (Cam Retard or Offset), DIST, EGR, PCV, TPS, CAT's (exhaust), MAP, ECT, CKP, CMP, etc. Most of these sensors can only be checked with a good engine capable scan tool in the hands of competent technician.

[j cAT]

Originally Posted by pikappkaz

OK, I have checked the voltage at the pump, 3 different times.

Before replacing any grounds or wires: 11.75V
After replacing the fuse and fuel relay: 11.75V
After rewiring the ground from connector to rail: 11.75V

Voltage at the fuse is 11.96V, so I don't think that slight drop is going to affect it, does it?

I'm still pushing 56psi pressure KOEO, I cannot get this value to rise.

Any thoughts, is this voltage the issue?

yes ,,with the engine running I WOULD EXPECT that the voltage would be 13.5-14.75 volts dc...

with the engine off you still should have at least 12volts..with the pump drawing some power..

the voltage drop from fuse to pump looks good..its just that the input to fuse appears low...check the battery grounds and other battery connections as well as getting the battery load tested just in case its loosing its capacity///a cell is going bad...

[pikappkaz]

Thanks, I'll give the battery a shot, but the testing was done with the engine off, so I'd expect the voltages to jump a little, I'd hope.