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i just had the in tank fuel pump replaced with a new one.. my car still will idle at about 750 then all of a sudden dip to under 500 and act like its going to die.. until i push the gas then it flys way up to 1000 even on the slightest touch...

i thought it might be a fuel pump, but after having the pre pump replaced the main pump still buzzes like crazy... do you guys think i should have the main pump replaced? or is it probably somethign else altogether? if i got a new main pump do you think it would have the same buzz as the old one thats in the car now? i thought the new pre-pump would fix the buzzing sound

oh by the way, where is the main fuel pump located? cause the buzzing is coming from the trunk, thers a box type thing behind the removable carpet, on the right side of the car, its pretty big gray plastic thing.. thats what buzzes.

[Edit by 88 535is south florida on [TIME]1110314558[/TIME]]

The main fuel pump is co-located with the fuel filter, above the L/R axle. The pump on the newer models have a rubber casing, and you can have a friend either jump the #1 or turn the ignition on/off a couple of times to see if it is working.

If your pump is good then other problem might reside with the FPR. Do a vacume test to see if it holds, if not then the diaphram is bad.

so the main one is above the axle in the trunk or below the body of the car... my car is an '88


[Edit by 88 535is south florida on [TIME]1110315346[/TIME]]

It is mounted above the axle in front of the gas tank to the floor of the trunk. You will have to raise the vehicle to get a good "look" at it though. Normally you can hear it buzzing, but if you are unsure, then do the touch test. If it only vibrates slightly, then it is either dead or pushing lightly. You should feel a defined vibration coming from the pump itself, just don't get the light vibration confused with the in tank pump.

ok what is this?



it sounds like the buzzing is coming from this gray box... its really loud you can hear it like standing 20 feet from the car

That is the vapor tank. It is used to "reclaim" any gas vapors from the main fuel tank and then return them once cooled. The left single hose, which you can't see, runs to the front of the car and gets filter at a charcoal canister and then "sucked" into the Boot between the AFM and TB. The other three lines you can see run down to the top or neck of the fuel tank. The buzzing could just be sound waves running up the hoses into this tank.

ok.. i guess i'll get the main fuel pump replaced... i dont even know if its related to the idle problem, but it should fix the buzzing correct? or is there anything else that can be associated with that, other than the pre-pump which i already got

If the main pump is inop then it would put a lot of strain on the pre-pump, that could be what is causing the hi-pitch vibration you hear. Do check the main pump, just don't go out a buy one if the old one is still good. The other thing you could do is test to delivery system, by removing the fuel line and see how much is being pumped. I think that it is around 1.7 liters a minute, but you would have to check out ole bently to be 100 percent certain.

[QUOTE="88 535is south florida"]ok.. i guess i'll get the main fuel pump replaced... i dont even know if its related to the idle problem, but it should fix the buzzing correct? or is there anything else that can be associated with that, other than the pre-pump which i already got[/QUOTE]

Bro, if you can use a screwdriver, you can replace the main pump yourself. It's not worth coughing up an hour of labor for such a simple job.

On a different note -- your problem doesn't sound like a fuel delivery problem to me. More like a bad ref sensor. A bad ref sensor may be causing your DME to futz with the timing. Testing the sensor is easy with a VOM, replacing it is about a minus 1 on the difficulty scale of 1 to 10...

And on yet another note, take a lesson from someone who has learned the hard way -- diagnosis via parts replacement is an expensive way to not find your problems. Get out the Bentley, follow the troubleshooting steps.

Bentley has specs for how much current the primary should be pulling. Its a quick easy check without disconnecting anything.

And the ref sensor might just need a good cleaning. It did wonders for my car.

dj


[Edit by djazz on [TIME]1110318972[/TIME]]

is it possible that the other day when i had a new throttle body put in.. (cause the old one broke) that they messed something up which is causing this irratic idle

is it possible that the other day when i had a new throttle body put in.. (cause the old one broke) that they messed something up which is causing this irratic idle

DING DING DING

Don't spend another dime on parts until you check the adjustment of the Throttle Position Switch (TPS). Assuming your '88is has a 5 speed trans, the TPS contains two micro switches. One switch tells the ECU that the engine is idling and causes the ECU to revert to a set of special maps for idle. The other switch is only used at Wide Open Throttle (WOT) and this signal tells the ECU to go open loop.

The adjustment procedure is in the Bentley.

Rich

yea my cars an automatic

The automatic cars still have the TPS and it needs to be properly adjusted for the idle not to fluctuate.

But if you'd rather spend hundreds of dollars throwing parts at your 5er versus making a simple adjustment. Knock yourself out, it's your money.

I'll even offer a few suggestions.

Idle Control Valve
All Rubber hoses in the intake tract
Air Flow Meter
Motronic ECU
Reference and Speed Sensor
Fuel Pressure Regulator
Main Fuel Pump
Injectors
CSV
Thermal Time Switch
Coolant Temp Sensor
Fuel Injectors

You could replace all these parts and still have your idle problem, which might be corrected by properly adjusting the TPS.

Have Fun

Rich

[Edit by Rich Euro M5 on [TIME]1110409887[/TIME]]


[Edit by Rich Euro M5 on [TIME]1110410117[/TIME]]