Flat spot on acceleration

#1

I need a little help from the SpecE30 guru’s.
I recently purchased an 1987 325 IS which I have plans to build out for Spec E30 racing. It is high mileage car 230k, but a local owned and garaged 2 owner that has been BMW maintained since new.

I have a flat spot between 800 and 1000 rpms. When cold the car revs like a champ (idle at 1000 rpm), once warmed up (idle at 800 rpm) the car is tough to get past the idle rpms to a higher rev. Like a flat spot that requires a bit of gas pedal manipulation to get past.

I have done full tune up - plugs, rotor, cap, wires, air filter, oil and filter, all new belts. I just adjusted the valves to specs. I also have been running Sunoco 93 octane and hi-tach fuel injector and valve cleaner through the last tank of gas.

Has anyone experienced a similar issue with the E30?

Thanks

#2

Sounds like it might be AFM (worn out trace for wiper arm). You can test it with AFM out of the car.

Regards,
Igor

P.S. Actually check this first - It could also be that your throttle cable is bit loose. With foot of the gas pedal, cable should have about 1 mm ‘free space’ between throttle body bracket and plastic screw stop on the cable. It’s an easy fix - tighten the white (yellow?) stop so that there is only 1 mm gap between it and throttle body bracket, and tighten the small nut that is in front of the plastic stop.

Another possibility is that there might be issue with your throttle body or throttle plate

#3

Check the TPS per the Bentley manual.

#4

Thanks for the replies and suggestions.
Splain Lucy - what is AFM and TPS?

Sorry but I am missing the acronym associations.

#5

AFM = air flow meter on the air intake
TPS = throttle position sensor

#6

john rock wrote:

[quote]Thanks for the replies and suggestions.
Splain Lucy - what is AFM and TPS?

Sorry but I am missing the acronym associations.[/quote]

AFM - Air Flow Meter. Location: Find housing that holds air filter; right behind it is funny looking box - that’s AFM. If you look at the second picture in this e30 timing belt and water pump procedure, AFM is right where letters ‘d.co’ are ( from e30world.com lettering). Or in fifth picture right where it says ‘radiator mount bolt’.

TPS - Throttle Position Switch. It’s located right under throttle body. If you follow the elbow or "L" shape hose from AFM towards the engine, next thing you will encounter is throttle body (it’s mounted on the engine intake manifold). As throttle plate opens and closes it rotates ‘swiwel’ inside TPS, which in turn gives signal to the ECU. TPS is a switch witch gives three positions to the ECU: idle, ‘driver pressed gas pedal’, and WOT (Wide open Throttle). ECU uses this information to access different fuel maps among other things. Form rest position, about first 10 % of the turn switch will indicate idle position, and about last 10% to max it will indicate WOT (‘driver pressed gas pedal’ is everything in between). Sometimes TPS can get out of alignment with throttle plate and not indicate correct position (i.e it migh be turned so it never reads idle position, and the car will never idle properly…).

While your problem might be TPS related, I wouldn’t think so because you said that the car idles properly. My money is still on throttle cable being loose or AFM, but I might be wrong…

HTH,
Igor

P.S> Took me 15 min to write this so Bruce beat me to the punch :slight_smile:

#7

Thanks, Igor.
Don’t worry about being beat by Bruce. I will reward you with double brew servings as a thank you at the next event . . .

These explanations are extremely helpful. I am planning to "investigate" while at the garage later this week. I will drop an email with results.

Thanks ALL for the help.

John

#8

Also test the coolant temp sensor. If you don’t have the Bentley manual. Get one. It is worth every penny.

#9

A bimmer with no Bentley manual - haha ??? you jest
I am about ready to purchase my second manual as I have already just about trashed the first one - in only 6 months of use.

Thanks for the tip on sensor

John

#10

Tonight I pulled and checked the AFM. It had full movement on the air flow sensor flap. I checked the ohms on the air temp sensor and it was within limits as per Bentley. I then adjusted the throttle cable which was way loose. Then I checked the coolant temp sensor and the ohms were within tolerance as per Bentley. After letting car warm up the idle is better but still has a bit of a flat spot.

I think it is time to see the local BMW doctor. I suspect a clogged injector and or deposit on intake valve.

Thanks to all for the suggestions.

John

#11

Good to hear that there is progress. To properly test AFM you have to measure resistance as the flap is being moved. "Flat spot" would be that for given flap movement resistance doesn’t change, and then as you move flap further it would ’ jump’ instead of consistently rising …

HTH,
Igor

#12

Did you check for a smooth change in resistance as the AFM vane moved?

#13

john rock wrote:

[quote]A bimmer with no Bentley manual - haha ??? you jest
I am about ready to purchase my second manual as I have already just about trashed the first one - in only 6 months of use.

Thanks for the tip on sensor

John[/quote]Wow…Mine is 14 years old now…cover came off, but otherwise intact…if a little greasy

#14

Does the car have a new cap/rotor/plugs/wires??

#15

I checked this and it seemed to be smooth through out the full range. As per Bentley I checked to see that there was full movement, no restrcictions and smooth movement.

#16

Yup. See original post for details of all work completed to date.

#17

john rock wrote:

Sorry…missed that part. Check the TPS…it may be failed where it thinks the throttle is closed even though it is opening and is still in idle mode but then the o2 sees lean and adds fuel and off she goes.

#18

Scott thanks for the tires.

Sean thanks for the radiator, working great and best of all - not leaking. This weekend I plan to try the next approach to the idle problem by "burning out any carbon deposits" - AKA - high revving the car by autocrossing at Ripken. I still have not tested the TPS. That will come next week. Thanks all for the tips and suggestions.

John

#19

I think I finally solved the flat spot issue. Re adjusting the valves helped a bit. However last week the latest copy of Bimmer mag came and there was a question from a reader with a similar issue with an E30. They suggested pulling the idle control valve and cleaning with carb cleaner. I pulled everything up to the intake manifold including the idle control valve and cleaned everything with carb cleaner a couple of times. I kept spraying until clear fluid can out instead of brown funky looking fluid. Tonight I put it all back together. When I fired the car up it started and ran smoother than it has since purchase. Didn’t take it out for road test but the mystery flat spot is gone.

Thanks for the help and suggestions.

John
http://www.cardomain.com/ride/2681791