fuel rate problem

[zig9765]

i have a 97 k2500 6.5l with a rough idle i hooked my snap-on modis and it is showing a fuel rate of 1 to 3 mm3 i unhooked the optical sensor the fuel delivery goes up to 10 mm3 so i put a new optical sensor in back down to 1 to 3mm3 anyone have this problem or can give me advice

[Hubert]

The fuel solenoid on the back of the IP controls fuel metering. The FSD or PMD drives the the electrical /mechanical metering solenoid. The optical sensor just measure time of the IP.

When you unhook the optical sensor it goes to a commanded default timing and fuel rate and will mask a fuel metering issue. Vs a measured and controlled fuel rate from the PMD/FSD and ECM controls.

The problem could be electronic in nature bad ground or voltage or a mechanical issue as in worn fuel solenoid "plunger" or aging IP.

Have you check fuel filter,lift pump, and fuel supply quality etc. Add some diesel fuel treatment and lubricity to the fuel 2 stroke oil or 30 wt non detergent. Then check voltage battery terminals and grounds etc.

Give us the age of the truck, general mechanical conditions, and a little history (mods etc) then other suggestions will follow.

[zig9765]

a little about my truck it has 248000 miles on it fsd cooler, intercooler, 4" exhaust, db computer, new injectors, dual eletric fans, i have never had a problem with this truck other then the dealer replace ip at 90,000 miles i just put a new fuel filter in i have plenty of fuel flow check it thru the water drain one other thing that is related to this problem the higher the resistor the rougher the idle and lower the fuel delivery with a #5 it idles ok but the fuel delivery is still low around 3mm3 with a #9 its around 1mm3 and its not stabile it goes to 0 then up to 2.9mm3 then to 1mm3 its all over the place but like i said its better with a lower resister but not right it will run smooth but low power do to the low fuel rate

[JohnC]

The reported fuel rate is only accurate if the correct resistor is installed. The correct resistor is determined when the pump is calibrated. Performance wise, the only thing the reisitor can do is trick the pump into delivering a small amount of extra fuel at full throttle. Below full throttle the fuel rate is determined by the PCM dependent on throttle setting.

The actual fuel rate required to idle should be in the 7-8 mm3 range.

Sounds to me like the pump is not correctly calibrated, or maybe (less likely) it requires a very low number resistor. I suspect your idle issues are related to the fact that the PCM and pump are not designed to control in the 2-3 mm3 range.

[zig9765]

this pump has been in this truck for years and this problem just started about 3 weeks ago

[JohnC]

Regardless, something has thrown it out of calibration. The PCM is trying to control idle. It is sending commands to the pump to fuel at a rate way below where the engine can run. The pump has to be delivering much more fuel than is being commanded. If it were really delivering 1-3 mm3 the engine would not run.

[Hubert]

I can't do it justice but the ECM controls idle and measures fuel rate with fuel solenoid closure time. It measures current draw to close. It uses optic sensor and crank position sensor for references. But it doesn't actually measure fuel rate so while it is saying 1-3 mm^3 fuel its probably more.

Fuel lubricity, air, or quality could be the culprit. The fuel solenoid plunger could be worn or gummed or etc. It could be electrical too.

The OBD system won't diagnose this very well unless a sensor goes out or a measure is so far out of range. Its probably just inside of being OK such that it doesn't code.

Here is an article explaining how the error can build and make a mountain out of of a mole hill for out of sinc control of timing of fuel rate control.

http://www.asashop.org/autoinc/oct2004/techtotech.htm

[zig9765]

hubert thanks for that link that explains what my truck is doing exactly im going to look into that. if that is my problem i do have a new solenoid and pludger

[Hubert]

I think that solenoid is set at close tolerances and not easily changed. The calibration resistor fine tunes it I believe.

I took one apart and and no way could reset it with normal tools. How are you going to do it? Do you have depth micrometer and ability to measure setup accurately? I think its around 0.005 not that tight if you have the tools. But I have never seen the procedure.

Before you take it apart try the double or triple dose of diesel treatment and some lube and see if it improves might help confirm its a mechanical issue.

I suspect a fair percentage of rebuilt pumps are cleaning, fresh fuel solenoids, plunger, FSD/PMD, and re-setup.

[JohnC]

If the solenoid isn't closing in a timely fashion, that surely would throw it out of calibration. Use the scanner to monitor solenoid closure time. Usually when it's that far out of whack it causes stalling, too.

Like Hubert said, try a heavy dose of fuel conditioner to see if you can free it up.

I know it's not recommended, and I don't know of anyone who has successfully replaced the fuel solenoid without a Stanadyne test bench. Could be wrong, but... YMMV