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Stumbling at highway speeds


pikappkaz
06-18-2009, 10:09 AM
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
06-18-2009, 01:44 PM
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
06-18-2009, 02:17 PM
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
06-18-2009, 07:36 PM
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
06-18-2009, 07:59 PM
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
06-19-2009, 07:13 AM
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
06-19-2009, 09:11 AM
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
06-22-2009, 02:17 PM
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
06-22-2009, 07:21 PM
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
06-22-2009, 07:21 PM
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
06-22-2009, 08:12 PM
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
06-28-2009, 05:00 PM
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
06-28-2009, 07:19 PM
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
06-28-2009, 11:21 PM
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
06-29-2009, 10:11 AM
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.

pikappkaz
06-29-2009, 07:11 PM
14.15 volts when running, not a battery issue.

rhandwor
06-29-2009, 07:24 PM
If it occurs on a hill I would suspect the EGR valve sometimes to much flow will cause this problem. After market EGR valves will cause if the proper shim isn't installed for this engine.

pikappkaz
06-30-2009, 08:52 AM
Wouldn't the EGR throw a code if it was having issues? I could be wrong there, no idea. I'll take a look at it.

Does anyone have the wiring diagram for the fuel pump? I'd like to test some other points just to be sure, still baffled on this one.

pikappkaz
07-02-2009, 02:11 PM
Anyone have any idea if the threads on the fuel filter are the same as the valve on the top of the fuel lines (where you plug in the fuel pressure valve? I still have low fuel pressure, so I thought that tapping in at the fuel filter and checking the pressure there would help to narrow some issues down. Is there any issue with doing this? I have no idea other than a clogged fuel line at this point, it is definitely a fuel pressure issue though.

j cAT
07-02-2009, 02:40 PM
Anyone have any idea if the threads on the fuel filter are the same as the valve on the top of the fuel lines (where you plug in the fuel pressure valve? I still have low fuel pressure, so I thought that tapping in at the fuel filter and checking the pressure there would help to narrow some issues down. Is there any issue with doing this? I have no idea other than a clogged fuel line at this point, it is definitely a fuel pressure issue though.

with the fuel filter match the threads to your fuel gauge ...this is a different thread...

at the store they may have adapter...

j cAT
07-02-2009, 02:47 PM
Anyone have any idea if the threads on the fuel filter are the same as the valve on the top of the fuel lines (where you plug in the fuel pressure valve? I still have low fuel pressure, so I thought that tapping in at the fuel filter and checking the pressure there would help to narrow some issues down. Is there any issue with doing this? I have no idea other than a clogged fuel line at this point, it is definitely a fuel pressure issue though.

on you fuel pump....,what was the part number ????

pikappkaz
07-02-2009, 03:03 PM
The fuel pump is P/N FG0088

j cAT
07-02-2009, 05:00 PM
The fuel pump is P/N FG0088

the 88 is for the 2door tahoe 5.7L

the 89/90 is for the 4door tahoe...5.7L...

pikappkaz
07-02-2009, 05:11 PM
I have a 2dr tahoe, so this is the correct fuel pump. I had to trick the dealership into telling me the correct part number per the VIN so I could order it elsewhere for half the price.

j cAT
07-02-2009, 05:24 PM
I have a 2dr tahoe, so this is the correct fuel pump. I had to trick the dealership into telling me the correct part number per the VIN so I could order it elsewhere for half the price.

usually rock auto has the gm part numbers..

cjttmyers
07-02-2009, 08:05 PM
I had the same issue on my '99 Yukon. Ended up being the distributor. The bushings were worn out and the shaft would move around at high speeds can cause the miss. Put in a new one and no more problem.

pikappkaz
07-03-2009, 11:41 AM
Any idea if there is diagnosis I can do on the distributor? Can I pull it and check the play manually? I'd really like to just throw one in there and find out, but I've already spent a ton on this problem

cjttmyers
07-03-2009, 12:40 PM
Unfortunately I found out the hard way. It finially got so bad it messed up the cap and rotor button, then it would not run at all. So I had it towed and the local shop changed the distributor. They are the ones that told me about the problem. They told me they used plastic bushings and that it was a common problem.

I do not know if you can check to see how much side play you have in the shaft. The only code I ever got from mine was a #3 cylinder miss. The problem did get worse over time and it took well over 2 years to finially die.

Like you, I spent a lot of time and money trying to find the issue. I finially gave in that I would have to take it to a shop. I just never set up a time to take it. I guess it set up a time.

pikappkaz
07-03-2009, 04:21 PM
Well, I managed to jack up the timing. Looks like a job for tomorrow. Any tips on how to get #1 to TDC? Pull all the plugs?
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j cAT
07-03-2009, 09:08 PM
Well, I managed to jack up the timing. Looks like a job for tomorrow. Any tips on how to get #1 to TDC? Pull all the plugs?
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#1 cyl up and distributor rotor at #1..mark wires///measure wires any way just to be sure there good with ohm meter..

with the bearing bad I would expect a new cap/rotor to not look new anymore..

pikappkaz
07-05-2009, 04:19 PM
OK, I've tried to get this timing correct, but I can't get the truck to start at all, after 3 attempts.

I'm following this guide: http://autorepair.about.com/library/a/1a/bl340a.htm

The question I have is in regards to getting #1 to TDC, the guide says there should be 2 timing marks on the crankshaft balancer. I can only find one, a line that goes from the front to the back. I don't see the other one that is supposed to be 90 degrees away from it.

I aligned the mark I can see up with the timing marks on the block, put the distributor back in, and I have the rotor aligned as it says in the guide.

What am I doing wrong? This thing is going to the dealer tomorrow if I can't get it.

rhandwor
07-05-2009, 07:08 PM
Hold your finger over no.1 spark plug hole with the plug out. Have somebody crank the engine when you feel air immediately stop. Then align the marks indicated in the picture. The rotor should be done as per instructions. Look at the rotor tip it should be close to no.1 plug wire.

j cAT
07-05-2009, 08:35 PM
OK, I've tried to get this timing correct, but I can't get the truck to start at all, after 3 attempts.

I'm following this guide: http://autorepair.about.com/library/a/1a/bl340a.htm

The question I have is in regards to getting #1 to TDC, the guide says there should be 2 timing marks on the crankshaft balancer. I can only find one, a line that goes from the front to the back. I don't see the other one that is supposed to be 90 degrees away from it.

I aligned the mark I can see up with the timing marks on the block, put the distributor back in, and I have the rotor aligned as it says in the guide.

What am I doing wrong? This thing is going to the dealer tomorrow if I can't get it.



before you remove distributor, remove the dist cap...rotate engine to rotor pointing at /near #1 plug wire would be on cap ,,,while checking the #1 piston is up all the way,,,then look at the timing marks,,should be close to zero.....then pull old distributor and install the new in exactly the same position,,,make some reference marks,to do this correctly...

since you have rotated this engine you are now going to install the distributor several times using reference marks a tooth at a time to find the correct position....

engine may be flooded also...

pikappkaz
07-06-2009, 06:29 PM
In a line of defeat, the Tahoe went to the dealer today to get the timing done correctly. $$ and it didn't fix the problem of stumbling on the highway.

He verified that the fuel pressure was low, low 50's when running. Unfortunately they didn't have time to run the full fuel system diagnosis. I guess its going to have to go back to them.

How difficult is it to take off the fuel hoses? I haven't taken a look at it, but assuming that all signs point to a clog somewhere in the fuel line, where should I start to check that?

j cAT
07-07-2009, 08:48 PM
In a line of defeat, the Tahoe went to the dealer today to get the timing done correctly. $$ and it didn't fix the problem of stumbling on the highway.

He verified that the fuel pressure was low, low 50's when running. Unfortunately they didn't have time to run the full fuel system diagnosis. I guess its going to have to go back to them.

How difficult is it to take off the fuel hoses? I haven't taken a look at it, but assuming that all signs point to a clog somewhere in the fuel line, where should I start to check that?

I disagree with the blockage clog theory...this would cause the pressure to drop when the engine starts...only...

your problem is low pressure even when the engine is not running..this could be injectors leaking large amounts of fuel,,,or a line leaking fuel....,fuel fiter leaking,,,fuel pump defective/not installed correctly....bad wiring inside the tank,,,ground return of the fuel pump,, fuel relay ....

the electrical with your posting appears good outside the fuel tank..

pikappkaz
07-08-2009, 03:45 PM
So you think that both fuel pumps were/are bad? The old one and the new one?

sasf
07-16-2009, 03:59 PM
hi, please let me know if you got the problem fixed with your tahoe, i have replaced all of the above, and still the same results as pikappkaz. i think there is a gremlin in there slowly pulling off plug wires while engine is underload. thanks

pikappkaz
07-16-2009, 04:24 PM
Is your fuel pressure low as well? Mine starts off low, and I have not been able to fix the problem. I have checked seemingly everything on the truck, and still no results. I show good voltage at the plug.

Does your fuel gauge flutter at all?

These things are nothing but headaches. I have given up trying to find the problem, I just have to be very light footed on the highway when I drive it.

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