Engine jerks at low constant gas.

J

johanaxelson

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Hi guys,

I have a little problem;

my engine (full 535 -91 conversion) has started jerking seriously at low RPMs, for example if I cruise at 2000 RPM with pretty much 0 pressure on the pedal. During acceleration it runs smoothly.

I just had it by a BMW shop who tuned the CO a bit and they claim there is no connection as CO only affects the idle. I'm not savvy enough to know.

Do you think the two could be connected as it started after the CO tuning?
I have also noticed the Air Flow Valve is pretty darn dirty and I've read that they sometimes need to be cleaned and may cause this kind of problem?

I think my engine has just the valve and not this sensitive little wire thing which seems reisky to clean.

Any tips on what the problem could be?

cheers,

Johan





air flow meter
 
Cleaning the AFM could help, or your idle control valve. (ICV) Do you have an intake leak? Sometimes the pipes, when moved, crack, causing a big intake leak, which would make it run poorly.
 
Two possibles right off the bat-- Failing fuel pump, or vacuum leak.

The guess is based on the fact that the problem wasn't there before, and has come on as a symptom.
 
Two possibles right off the bat-- Failing fuel pump, or vacuum leak.

The guess is based on the fact that the problem wasn't there before, and has come on as a symptom.
Click to expand...

But if it was the fuel pump or fuel filter, wouldn't it have trouble stumbling when accelerating, rather than when at low RPMs?

Oh, what about ignition components?
 
Well, the whole exercise started with a leak in a vacuum hose which I fixed. That's why I took it to BMW for a quick tuning just in case....

Just ran badly before that, no jerking. Used tons of gas and th eplugs sooted up.

And if the pump is failing I shouldn't be bale to accelerate without the jerking? I go up to 5500 and it runs cleanly albeit a bit weak.

It can't be that the CO screw is set to tight so it doesnt get enough air?
I googleda bit and that's what it seems to do, control the air?

Fuel filter, spark plugs and rotor are new. Ignition cables 2 years old.

And as for the fuel pump, it is pretty noisy but sounds "OK". Haven't heard another one in a long while for reference.
Is a new one expensive?

/J
 
Yes, it's possible, but a failing fuel pump will sometimes do strange things (i.e.: Illogical behavior). Also, both of these suggestions require the least amount of work: It should be fairly easy to check the fuel pressure at the rail, and tracing vacuum leaks with your buddy WD-40 is easy enough.

Tracing ignition issues may cost you more, as you may buy stuff you don't need. Obvious checks sure, but I'd try the other stuff first.
 
I assume its Motronic, correct? Any idea how they tuned the CO? Was it by adjusting the AFM (most likely)? I have limited knowledge of how to calibrate the Motronic AFM, but I do know it changes more than just the idle mixture. Sounds to me like your running lean. I would try another AFM or find someone who knows how to properly adjust yours.
 
Check for vacuum pipe leaks as allready mentioned

I had a similar problem a few years ago with a 528 I'm not sure that the two are connected because you say that your problem started straight after a "tune up" however my problem was down to the air flow meter, one of the tracks was worn at a point that corresponded to 70 mph which is the speed limit on the motorway in the UK so obviously my car had spent a lot of time tramping up and down the UK. I'm curious as to what exactly this garage you took your car to actually did to "tune it up" as there isn't any thing that you can do to a Motronic system except set the base CO and that only effects the idle. Have a good look at your AFM (remove it if neccesary) and see if it looks like the black plastic cover has been prised off. If it has then it could be that they have done this to alter the preload on the spring that works against the air flap, in doing this it is possible to alter the fuel curve through the rev range and if they have then it may be possible that they could have inadvertantly dammaged a portion of the track which would lead to the symptoms you describe.
These tracks are divided into segments so if one becomes damaged it doesn't effect the vehicle through the whole rev range.
Another possibility is an incorrectly adjusted throttle switch, this is not a potentiometer as some people belive it is simply a switch (actually it's 2 switches) to tell the ECU that: a) the throttle is closed : b)the throttle is wide open.
I don't think it will be a problem with the fuel delivery if you have described the symptoms correctly because your fault is at light throttle and you say it pulls strongly when its opened up.
If you're cruising at 2000 RPM and you change down a gear or two so the revs increase but your still cruising (very light throttle)does it stop jerking? If it does I'd put money on it being the AFM.
Hope this helps.

Gazzol
 
thanks again.

some replies:

yes, it's a Motronic and it does feel like it is running lean.

All they claim to have done is adjust the CO.

When I increase the revs, say above 3000 it seems to stop jerking but is a bit weak.

I will open it up and have a look to see if there is anything obvious and clean what I can.

I guess I can also play around a bit with the CO screw to see if somethign happens. Don't think I can damage anything?

/J
 
Motronic FI is pretty robust, plus the sensors are as a whole pretty reliable. It basically needs:
Amount of air entering the engine
Position of the crankshaft
Coolant temperature
throttle position

Engine timing is all done electronically with a "fixed distributor". Early Motronic uses a different system to calculate and fire than later ones

Anything that can "influence" the system will effect it so check the following, I know some have been mentioned before so sorry in advance for the repeats:

1...Idle control valve aka air bypass is working, not stuck open.
2...Air flow meter - make sure the flap actually moves smoothly across the whole range of it's travel.
3...No air leaks down stream of the air flow meter, don't forget the vacuum line to the servo, inlet manifold, injector seats, crankcase breather!
4...Output from air flow meter (resistance) should be linear.
5...Check all FI connectors (break and make) are clean.
6... no broken wires, earth (ground connections) clean
7... fuel filter renewed recently?
8... pump output correct - approx 900cc/min
9... Pressure regulator opertating correctly? (need to measure fuel pressure).
10.. Throttle position switch operating correctly, positioned correctly?
11.. Depending on where the Motronic sensors are, Flywheel early, front pulley, late make sure the gap between the sensor and the markers is correct, sensor mounted firmly.
12.. Coolant temperaure sensor output
13.. Assuming you have a cam driven Motronic diistributor, unscew cap and check rotor arm and cap for tracking, cracks or oil contamination.

Careful checking and work with a multimeter will certainly help. Once your happy that the basic system is fine, then you can start to twiddle with the CO etc


My scrapper M535 used to "hunt" at idle, revs going up and down, and was a bit "flat" at mid range revs, all down to air leaks - bad inlet gasket and one dodgy injector seat - bug*er to track down
HTH
MAlc
 
Thanks Malc,

long list. Should keep me busy over the weekend :)

And I did find a Haynes manual for the car that I could download. Tons of info in there too about what to measure and check.

/J
 
Air/fuel problem..

Sounds like the classic vacuum leak symptom to me. If I were trying to fix it I would start with the air leak hunt. Have fun!

Dan
 
looked inside my AFM and it was pretty greasy, the little wire thingy had thick soot on it so I thought "yay, found the problem!". I then cleaned it off and remounted and it runs a little bit better. Still jerks though.

Brought out the multimeter and checked the temp meter and movement values of the valve on the AFM and it looks ok.

More tomorrow, will try to walk through the things I can measure in the haynes book and see what it tells me.

New AFM is like $500 so if I don't have to buy one.....
 
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