'96 Seville stumbles on acceleration

[KHE]

My '96 Seville began to stumple on acceleration. There are no trouble codes stored in diagnosis nor is the "Service Engine Soon" light illuminated. I began to suspect the fuel pump may be on its way out so I hooked up my fuel pressure gage to the fuel rail and taped the gage to the windshield. The fuel pressure at idle was 40-42 psi and upon acceleration, it jumped to 50 psi. When I let up on the accelerator, it dropped to 39-40 psi. Is this normal or does it indicate that the fuel pump is on its way out?

I removed the fuel filter and was able to blow through it just as easily as a new filter so I feel the existing filter is OK.

I looked in my factory service manual and the table of contents states fuel system information is in section 6C but there is no section 6C....

[Ranger]

If there is no section "C" then it is usaualy in the "other book". Jusy off the top of my head I think your fuel pressures are a tad on the low side. But if my understanding of the system is correct it should remain relatively steady. That is what the FPR is for.

[KevinW]

If there is no section "C" then it is usaualy in the "other book". Jusy off the top of my head I think your fuel pressures are a tad on the low side. But if my understanding of the system is correct it should remain relatively steady. That is what the FPR is for.

Kevin, 6C is also missing from my set. I believe the fuel pressure specification is 48-55 psi, while the fuel pump is running, engine off (no vacuum at the regulator), after air has been bled from the gauge.

Larry, the FPR varies fuel pressure in response to manifold vacuum in order to maintain a relatively constant pressure differential across the injectors. When there's high manifold vacuum (closed throttle), less fuel pressure is required to achieve a desired flow rate. Some fuel systems run at a 'constant' pressure and have PCM code that modifies the pulsewidth in response to manifold absolute pressure.

[KHE]

Kevin,

It sounds like my pump is on the weak side. I'll recheck the pressure with the engine off, pump running. When I floor it, the pressure is within the range you specified. I want to make sure the pump is bad before I drop the tank.... Is there anything else that you'd check or would you just replace the fuel pump?

The FPR was replaced less than 3000 miles ago and I checked for presence of fuel in the FPR nipple and there was none.

Larry,

There is NO section 6C in either volume 1 or volume 2.... I searched through section 6E unsuccessfully looking for the spec...

[bigfoo]

I have the FSM for my 99 which should be relatively exactly the same as yours

It says

ignition off

air off

connect bleed hose

turn ignition on

bleed air

ignition off

remove bleed hose

connect gauge

cycle ignition every 2 seconds until you get higest reading

should be 48-55 PSI

it says after turning the ignition on the fuel pump runs for 2 seconds which is why you cycle it

it also says wait 10 minutes after you get the highest value and see if the pressure drops more than 5 psi in 10 minutes (it shouldn't)

If you want I can email you images and stuff

[KHE]

Thanks for the info. I seem to remember that the pressure with the key on and engine off was about 42 psi but I want to recheck it to confirm. I want to be absolutely positive that I have a bad fuel pump before I drop the tank and replace it.

[bigfoo]

ya it says you might have to turn the key on and off several times because the pump only stays on for 2 seconds.. so turn it on, wait, turn it off, turn it on wait, turn it off several times to pump up the pressure and it should reach between 48-55 and it should not drop more than 5psi in 10 min.. that's my synopsis of the 500 word page lol

[Ranger]

If there is no section "C" then it is usaualy in the "other book".  Jusy off the top of my head I think your fuel pressures are a tad on the low side. But if my understanding of the system is correct it should remain relatively steady. That is what the FPR is for.

Larry, the FPR varies fuel pressure in response to manifold vacuum in order to maintain a relatively constant pressure differential across the injectors. When there's high manifold vacuum (closed throttle), less fuel pressure is required to achieve a desired flow rate. Some fuel systems run at a 'constant' pressure and have PCM code that modifies the pulsewidth in response to manifold absolute pressure.

Kevin,

I thought I remembered Guru once saying that the fuel pressure should remain relatively constant. I was in response to me asking him for an explanation of what the FPR does and how it works. As you can imagine it was quite detailed but I can't find it. I did however find the following post from him. Don't get me wrong here, I am not argueing with you as I suspect you have a better understanding of the system that I. Just trying to explain why I said what I said.

http://caddyinfo.ipbhost.com/index.php?sho...l=fuel+pressure

[JimD]

Here are the words from UNCLE on FPR theory of operation:

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Subject : RE: how does FPR work?

MessageDate : 1/10/2004 12:00:52 AM

Posted By : bbobynski

Cadillac Year : 2003

Cadillac Model : 05 Bonneville GXP

Message : The fuel pressure regulator works pretty good...LOL

Seriously....The fuel injector is basically just a valve that is

opened by an electric solenoid. There must be some fuel pressure on

the injector so that when it opens the fuel will be forced thru. The

pressure that the system was designed to operate at varies depending

on the injectors used, the system, the engine, the fuel pump, the

fuel, etc.... Typically port fuel injected systems operate somewhere

in the range of 40 to 60 PSI as a general rule of thumb.

The first job of the fuel pressure regulator, then, is to maintain

this basic level of pressure on the system. Since the fuel rail

supplies fuel to each of the injectors around the top of the engine

if the fuel pressure in the rail is controlled then each injector

will see the same pressure.

Basically the fuel pressure regulator functions by bleeding off

excess fuel back to the tank to maintain the desired pressure. That

is why there are two fuel lines...a supply line and a fuel return

line. The supply line is always under the pressure that the

regulator is regulating at and the fuel return line is not under any

pressure, per se, it just routes the fuel back to the tank.

The FPR does this via a poppet valve and seat that is controlled by

a a spring. The fuel pump supplies a constant flow of fuel to the

fuel rail and the FPR is at the end of the rail. The pressure in the

fuel rail that will unseat the check valve is determined by the load

of the spring behind it. If the opening of the poppet valve or check

valve is 0.5 inches square inches and the spring is compressed to a

value of 20 pounds then a fuel pressure of 40 pounds per square inch

(40 PSI) will force the valve open and the excess fuel will escape

and travel down the fuel return line to the tank.

The fuel pressure regulator on the port fuel injected engines, such

as the 4.9 and the Northstar, controls the fuel pressure in the fuel

rail so that the pressure drop across the injector is always

constant. That is, the pressure drop from the fuel inlet of the

injector to the tip of the injector, must always be constant so that

the injector pulse width delivers a predictable amount of fuel. The

"pulse width" is the amount of time the PCM holds the injector open.

It is usually measured in milliseconds. For instance, it would be

common to see an injector pulse width of 5.0 milliseconds.

This is important because the whole basis of the fuel injection fuel

delivery system is that the PCM can calculate how much fuel to

deliver and then open the injector for a specific amount of time to

allow that quantity of fuel to enter the port.

You need to understand this because the only way the injector can do

this is if the fuel pressure drop across the injector from the rail

to the tip is constant. This would be relatively easy if the

injector tip was in a constant pressure environment. It isn't in a

port fuel injected engine as the tip of the injector is exposed to

manifold vacuum. The manifold vacuum changes constantly with changes

in the throttle, load, RPM, etc....so...the fuel pressure must

constantly change to keep the pressure drop or the pressure

differential across the injector constant.

Since the simple spring in the fuel pressure regulator is not

capable of comprehending any changes in vacuum the spring load is

modified by a diapragm in the FPR. This diaphragm helps modulate the

spring load to the check valve when vacuum sucks or pulls on it. In

this fashion, when vacuum from the intake manifold pulls on the

diaphragm it will change the load on the spring seat and the

resulting fuel pressure required to unseat the valve will change.

That is why there is a vacuum connection to the fuel pressure

reguator. That is also why, if you put a fuel pressure gauge on the

fuel rail you will see a fixed, constant fuel pressure when the key

is turn on and the engine is not running. When the engine starts,

and manifold vacuum comes into being, the fuel pressure will now

fluctuate, relative to the intake vacuum at the time.

Since higher levels of vaccum in the intake tend to "pull" or "suck"

fuel from the injector tip, there needs to be less positive fuel

pressure on the back side of the injector to deliver the constant

fuel pressure drop across the injector. As the vacuum drops to zero

(as it is at wide open throttle..the WOT condition) the positive

fuel pressure on the back side of the injector must increase to keep

that pressure delta across the injector constant. And you will see

the fuel pressure go to maximum when the vacuum is zero.

In this type of system the fuel pump just dumbly pumps fuel at a

constant volume to the engine all the time. It must pump enough fuel

to be able to keep the fuel pressure constant even at maximum power,

when a great deal of fuel is flowing thru the injectors and into the

engine. This means that at idle, when very little fuel is flowing

thru the injectors, most of the fuel is just bypassing the system at

the FPR and going back to the tank. It just circulates constantly.

This also explains why the engine will run poorly when the fuel

pressure regulator acts up. If it sticks open and bypasses all the

fuel to the tank then no pressure will build up and the engine will

not run...since no fuel will be pushed thru the injectors. If the

diaphragm in the FPR, that separates the fuel from the vacuum

chamber, starts to leak then the fuel will be sucked into the intake

via the vacuum line and will enter the intake manifold. This will

cause very rich operation and poor performance, poor fuel economy,

hard starting, etc. will result.

Hopefully this sheds some light on what the FPR does. It is a

relatively simple device but it performs a rather complex function.

If you want to understand how the system works it is important to

understant the reason the fuel pressure must do what it does....and

it is the job of the FPR to control the fuel pressure accordingly.

This is the description of the classic port fuel injection system

typified by the systems seen on the 4.9 and Northstar. The systems

have evolved in the early 2000's to "returnless" fuel systems. These

systems are much more complex in the control side of the electronics

but simpler mechanically as they do not have a fuel return line, the

fuel pressure regulator is fixed and does not "see" vacuum. The

complete explanation of how this is possible will wait for another

day, but, be aware that there are variations on the classic FPR type

systems in production today. The same logic applies as is described

above as to how they work...they just have a lot more electronic

bells and whistles allowig elimination of the fuel return line.

***---REPLIED TO MESSAGE BELOW---***

could someone explain exactly how FPR works

how many openings does it have

Parent Message

how does FPR work? - michael w smith (2) - 1/9/2004 10:41:54 PM

Replies

[bigfoo]

Ya that about sums it up. If you want to add more fuel all you have to do is change the injectors and/or increase the pressure / bigger fuel pump/ different FPR.. helps for aftermarket mods..

[KevinW]

When I floor it, the pressure is within the range you specified.

Kevin, testing the fuel pressure under load (WOT at peak HP) is a more rigorous test than what the manual specifies (engine off). If it is close to this range under those conditions, the fuel pump is probably fine. Have you checked for any obstruction at the MAF sensor screen? How does it run when the MAF sensor is unplugged? Is the stumble related to the rate of throttle application?

Larry, the link you posted concerns diagnosing a severe driveability problem. If the fuel pump cannot deliver the required volume, the pressure will drop-off significantly (say 45 psi down to 20) once load is applied (opening the throttle off idle in that case).

The fuel pressure should remain relatively constant while under continuous WOT, but there will be some variation under part-throttle conditions in systems with a vacuum referenced regulator.

[KHE]

Kevin,

The stumble appeares to be worse when the rate of throttle application is fast - ie punching it from a stop or passing on a two-lane road but it does stumble under moderate acceleration just not as bad. I don't know why a trouble code was not set.

The pressure holds at 50 psi during hard acceleration. Logan suggested blocking off the return line slightly to see if the stumble goes away. I am going to try that along with inspecting the MAF screen.

Last night, I swapped each of the four coils with one known good coil off another car (3800 V-6) and the stumble did not go away. If you have any other ideas, please post them.

[Scotty]

It is sudden so maybe a bad load of gas with some water in it?

[KevinW]

The stumble appeares to be worse when the rate of throttle application is fast - ie punching it from a stop or passing on a two-lane road but it does stumble under moderate acceleration just not as bad. I don't know why a trouble code was not set.

Are you able to drive through the stumble or is it more or less constant while accelerating? How is it at a constant road speed (e.g. 4th gear, 50 mph, VCC locked)?

Scotty raised a good point about fuel quality. Did the problem coincide with a recent fill-up?

It may help to use a scantool to monitor some PCM parameters. If the problem occurs only on throttle transients, you could verify the TPS readings.

[PAUL T]

What about spark plugs or plug wires arcing?

[Scotty]

You know I read an article this morning about the ethanol in gas creating problems in boat motors clogging the gas filters. Boats and jet skis are stalling out all over supposedly. I know you checked the filter already. However, they say that it is affecting boats as 1) the gas tends to sit longer and 2) moisture. Both of these factors supposedly cause the ethenol to separate and create a waxy like deposit clogging the gas filters. I don't know how accurate this is, it was in a local paper. I wonder if fuel injectors could be fouled or disrupted by this stuff, just a thought

[bigfoo]

My 99 backfires rarely if I smash the gas pedal down real fast.. every so often it stumbles but not much.. usually happens when it downshifts right after i slam the pedal say going 30mph and i slam it and it drops into 1st and rpms go up and it stumbles or backfires occasionally.. happens maybe 1 out of every 50 times I do that so I haven't really looked into it much but it still bugs me. If it was carubuerated it wouldn't bother me at all lol

[Scotty]

maybe 1 out of every 50 times lol

Uhhh that would be 2% of the time you are unhappy with the pickup.. and it bugs you I don't think I have slammed the pedal 50 times at ANY MPH since I own my car Boy am I a wimpy driver

[bigfoo]

ahaha I do it all the time and it doesn't seem to change the pickup much it just make a noise or stumbles for a 10th of a second.. I tried changin the plugs and wires and it did it once since I did that so i guess that wasn't it. maybe a coil pack is intermittently going bad or a valve is sticking I dunno.. maybe an exhaust leak.. so hard to track down when the engine is PACKED in so tight.. ugh :/

[Scotty]

so hard to track down when the engine is PACKED in so tight.. ugh :/

I know how much this has bothered you and all the work you have done to track it down. Do you have a laptop, maybe hooking it up and watching real time data when it happens will narrow it down. How about this idea, you are using oil badly and I think the discussion was that the rings could be frozen, suppose you are getting one cylinder losing compression intermittantly on a compression stroke when you kick it.. what do you think about that? It would be hard to diagnose that I think.

[KHE]

The stumble appeares to be worse when the rate of throttle application is fast - ie punching it from a stop or passing on a two-lane road but it does stumble under moderate acceleration just not as bad.  I don't know why a trouble code was not set.

Are you able to drive through the stumble or is it more or less constant while accelerating? How is it at a constant road speed (e.g. 4th gear, 50 mph, VCC locked)?

Scotty raised a good point about fuel quality. Did the problem coincide with a recent fill-up?

It may help to use a scantool to monitor some PCM parameters. If the problem occurs only on throttle transients, you could verify the TPS readings.

I am able to drive through the stumble - the car will cruise fine it just stumbles on acceleration. It acts like there is water in the fuel so I added some isopropyl alcohol to the tank but it didn't seem to help.

My wife doesn't recall if the symptoms occurred with a recent fill-up but it could be.

[bigfoo]

so hard to track down when the engine is PACKED in so tight.. ugh :/

I know how much this has bothered you and all the work you have done to track it down. Do you have a laptop, maybe hooking it up and watching real time data when it happens will narrow it down. How about this idea, you are using oil badly and I think the discussion was that the rings could be frozen, suppose you are getting one cylinder losing compression intermittantly on a compression stroke when you kick it.. what do you think about that? It would be hard to diagnose that I think.

Yea it could be that, and it could be bad gas I dunno..

I've tried everything except pouring top engine cleaner in the spark plug holes.

I have a laptop and I'm thinking of buying that obdii thing and software for it and then I should be able to monitor everything.. maybe it's as simple as a bad o2 sensor or something. Maybe if I find out what it is, it will be the answer to KHE's problem also.. because it only does it when i hit the pedal 'fast' like he said :>

[Phillip]

Paul asked about plugs and wires but I haven't seen anything in this string to indicate that they have been checked. My first inclination is to check for a spark leak. It may idle and cruise fine but you will certainly feel a misfire under load. If you haven't done it already, check the plugs and wires. I recently had similar symptoms and traced it back to carbon tracks on one of the plugs. As far as I can tell it was caused by a flaw in a brand new spark plug wire boot.

[KHE]

After driving the car this weekend, the stumble appears to be a misfire. It will misfire on acceleration and occasionally at idle. I misted the plug wires with water and there were not any sparks. Tomorrow evening, I plan to remove and inspect the front four plugs.

I would have thought that the P0300 code (engine misfire detected) would have set.

[Phillip]

It would be nice to get a code everytime you have a misfire, but for intermitant misfires, you propably won't see a code. The computers are great but there is still room for patient diagnostics. I would carefully inspect all the plugs and wires. If there isn't anything obvious, you may have to replace them anyway. Do you still have the originals installed?