Rough Idle/Loss of power under load [M30b32]

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Tazzarti
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Location: Ontario, Canada

Rough Idle/Loss of power under load [M30b32]

Post by Tazzarti »

hi guys,

I have a 1981 633, and Ive been trying to get the engine to run smoothly for 3 years (as well as rebuild the rest of the car) but at this point Im stuck. Ill go over what ive done and the timeline as it may happen that something I fixed early on has now become defective since and Im overlooking it.

The basic symptoms are a rough idle, which might missfire on occasion, but the engine vibrates alot until around 3k rpm, and is smooth in neutral up to 6kish. This has been happeneing since I first fired her up. The second issue (not sure if they are related or not) is that the engine looses power at around 5.2k rpm while under load. Just revs up and goes no further, but if I go downhill she does pop up.

So the history:
This car had sat for 10 years and initially the engine ran with ether, but it had fuel issues. I replaced every part of the fuel system (tank pump, fuel pump, fuel filter, new lines, new injectors and fuel pump relay). After that the engine fired up for the first time but had a bad missfire and had lots of vibrations. Then I tried a replacement computer, new o2 sensor, new temp time switch (cause cold start didnt work), new wires, plugs, rotor and rotor cap. The electrical components made the engine run smoother at high rpm but nothing fixed the idle. Then I adjusted the valves (which I hope I did right) and checked the electrical values of the MAF which looked okay. The compression readings were all in spec 4 cylinders 160-175spi, and the lowest being 140 and 150, both going up 10 psi with oil added to the cylinder. All new vacuum and air lines and a guage the previous owner installed it showing good vacuum. Recently I put in a new fuel pressure regulator and that seemed to improve the missfire on idle but not the vibration.

So all i really have left to replace is the coil, and maybe change out the spark plugs again as they got fouled from running rich after i first replaced them (before changing the O2 sensor) but I have run some cleaners through and they seemed to be getting clean when I checked.

Any advice would be great, even suggesting I take a second look at things that I have already done.
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Hefeweizen
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Re: Rough Idle/Loss of power under load [M30b32]

Post by Hefeweizen »

I am afraid but the bad news seems to be the catalytic converter is clogged. My son's 6er was idling rough and you could crank up the RPM without any issues but heaven forbit, try to drive the car and he died (Car not my son). Replaced the cat and all went well
Rainer
03 330Xi (DD)
85 635CSi (RIP 3 May 19)
80 633CSi (Red Euro, Project)

"If you think a good European mechanic is expensive, try a bad one!"
Tazzarti
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Re: Rough Idle/Loss of power under load [M30b32]

Post by Tazzarti »

I wish it could be that easy, but I do not have a cat anymore actually :P straight pipe up front, Billy Boat performance exhaust in the rear (so just a muffler)
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Brucey
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Re: Rough Idle/Loss of power under load [M30b32]

Post by Brucey »

if the cylinders with low pressure were neighbouring cylinders, it might be that the head gasket was starting to blow, cylinder to cylinder, rather than just worn rings or something. The compression figures may have become worse with use regardless of cause, so check them again,

If the idle is terrible then (by pulling injector connectors or plug leads) you can diagnose which cylinders are misfiring worst.

Air leaks are common (inlet stub gaskets can even fail but there are many other causes) and if there is a problem with one or two cylinders the closed loop control will tie itself in knots trying to make a decent average mixture strength.


So those are the things that I would concentrate on first.


cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Hefeweizen
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Re: Rough Idle/Loss of power under load [M30b32]

Post by Hefeweizen »

Tazzarti wrote: Thu Jun 28, 2018 10:11 pm I wish it could be that easy, but I do not have a cat anymore actually :P straight pipe up front, Billy Boat performance exhaust in the rear (so just a muffler)
Glad to hear that! Sorry it was not that easy :(
Rainer
03 330Xi (DD)
85 635CSi (RIP 3 May 19)
80 633CSi (Red Euro, Project)

"If you think a good European mechanic is expensive, try a bad one!"
Tazzarti
Posts: 44
Joined: Fri Sep 26, 2014 5:49 pm
Location: Ontario, Canada

Re: Rough Idle/Loss of power under load [M30b32]

Post by Tazzarti »

Brucey wrote: Thu Jun 28, 2018 11:24 pm if the cylinders with low pressure were neighbouring cylinders, it might be that the head gasket was starting to blow, cylinder to cylinder, rather than just worn rings or something. The compression figures may have become worse with use regardless of cause, so check them again,

If the idle is terrible then (by pulling injector connectors or plug leads) you can diagnose which cylinders are misfiring worst.
I re-did the compression test with the following results:
1~152psi
2~150 psi
3~139
4~150
5~142 ( I think the gauge was slightly leaky in this cylinder)
6~155psi

I had very consistent measurements across a few trials. The spark plugs all looked good except one (cylinder 2 had a dirty, and long story short, ngk plug vs the boschs' that are in the rest). I replaced this with a fresh ngk I had on hand, but put the ngk in 4 and the bosch in 2 (forgot to reinspect cylinder 2 and 4's plugs before I left).

After that I started pulling plug wires and I believe that cylinder 6 is causing the missfire. (confirmed at idle ~750rpm, and @ 2000rpm).

So being at the end of the fuel rail could this be a fuel pressure thing? Since its cheap ill replace the fuel filter again.

Thanks,

Tazz
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Brucey
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Re: Rough Idle/Loss of power under load [M30b32]

Post by Brucey »

just to be sure, the conventional way of numbering cylinders is from #1 at the front (ancillary belt) end of the engine to #6 at the rear (bulkhead/firewall) end of the engine.

In most cases fuel enters the fuel rail near the #6 position and leaves (via the FPR) at the #1 end. There should be a copious flow, so that the fuel discharged through the FPR and returned back to the tank is 1-2 litres a minute or something. This is easily tested for by using a jumper wire (or manual AFM flap opening on early L-jet systems) to run the fuel pumps and discharging the fuel from the FPR exit into a container for a set period of time.

Given that compression figures are good on the misfiring cylinder (the one that when the plug lead is pulled makes least difference to the idle), you would have to have a fault that is specific in some way to cylinder #6. Possibilities include;

1) air leak on #6 intake (eg bad gasket or lower injector 'o' ring)
2) exhaust gasket leak on #6 (common here but doesn't usually cause a bad misfire on one cylinder only)
3) faulty injector (if injector is sticky this can produce a misfire at idle only)
4) blocked inlet to fuel injector (but this ought to produce a misfire not only at idle)
5) ignition fault specific to #6 (could be any part from the distributor cap to -and including- the plug)
6) locally abnormal running temperature for #6 cylinder (e.g. clogged coolant passages in cylinder head)

Note that under closed loop control the average mixture strength ought to be stoichiometric. However if one cylinder has a misfire for any reason, that will (irregularly) throw both unburnt fuel and oxygen towards the O2 sensor which may cause the fuelling to vary in such a way as none of the cylinders fire perfectly regularly.

Other clues are that ignition faults generally get worse at mid-throttle but if there is a gasket leak (inlet or exhaust) the effects tend to get less severe at mid-throttle openings.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tazzarti
Posts: 44
Joined: Fri Sep 26, 2014 5:49 pm
Location: Ontario, Canada

Re: Rough Idle/Loss of power under load [M30b32]

Post by Tazzarti »

Brucey wrote: Mon Jul 09, 2018 6:48 am just to be sure, the conventional way of numbering cylinders is from #1 at the front (ancillary belt) end of the engine to #6 at the rear (bulkhead/firewall) end of the engine.

In most cases fuel enters the fuel rail near the #6 position and leaves (via the FPR) at the #1 end. There should be a copious flow, so that the fuel discharged through the FPR and returned back to the tank is 1-2 litres a minute or something. This is easily tested for by using a jumper wire (or manual AFM flap opening on early L-jet systems) to run the fuel pumps and discharging the fuel from the FPR exit into a container for a set period of time.
Yes, 6 is at the back. I knew the fuel entered at the back but someone kept telling me the fuel had to go through the FPR first and made me second guess myself :S (but alas i was right! ahahaha....)

I will inspect the injector/intakes for that cylinder next time I am there.


As always, thanks for your input.
Tazzarti
Posts: 44
Joined: Fri Sep 26, 2014 5:49 pm
Location: Ontario, Canada

Re: Rough Idle/Loss of power under load [M30b32]

Post by Tazzarti »

Hoping to bump;

I checked the throttle position sensor, and something seems to be off.
https://imgur.com/pZQRD2O
Image
In the electrical diagram it says that pin 3 is closed and 2 is open. But on my car this is reversed. This reverse pin out seems to be the case for the cars from 77-79 (not 80 which apparently has a different sensor) then it is flipped for the 81 year.

I checked the part number and its a bosch 0 280 120 302 which as far as I can tell is the same as the BMW part specified in real oem:13631273265.

I am a little confused by this (maybe the switch is just broken)? Anyways, if this is the case what would be expected from the L-Jetronic unit if its getting the wrong input from this sensor? ( nothing seems to change when its unplugged).

I was also hoping someone with a similar MAF take some measurements of theirs for me. If possible all the combinations possible between pins 9, 8, 6 and 27 (+ bonus points for the ambient temperature for the resistor on 27).

Thanks
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