Author Topic: Air Fuel Ratio - running lean - help! (Read 7467 times)

finite · « **on:** April 02, 2009, 03:25:55 PM »

Hi all,

I've just finished building my track car for the nurburgring in a weeks time and have been running her around to iron out the last bugs. I've been checking the A/F ratio with a wideband for the last couple of days, seems i have an under-fuelling problem.

On continual load (cruising) the A/F is showing around 15.5 to 16
On WOT A/F is around 17

I popped the plugs out and they look clean, light brown in colour with some white pitting.

She feels sluggish under acceleration and will often stall at idle when coming down from acceleration or cruising. I've replaced the ICV already and done the TB heater plate removal, replacing all the vacum hoses and ICV hoses in the process. Full thread's here:

http://www.m42club.com/forums/showthread.php?t=7825

Can anyone help point me in the direction of what might be causing this running problem? I've only got a couple of days to sort this out before we head of to germany!

Danny707 · « **Reply #1 on:** April 02, 2009, 03:51:47 PM »

Vacuum leaks.

tjts1 · « **Reply #2 on:** April 02, 2009, 04:51:13 PM »

Does your car have a stock O2 sensor in the exhaust? I know some M42s sold in europe didn't and some did depending on model year and market. Vacuum leaks is a possibility but I would also suspect old worn out injectors, low fuel pressure or fuel filter. You can safely up the injectors to 22 or 24# injectors.

1991 E30 M42 · « **Reply #3 on:** April 03, 2009, 12:09:18 AM »

When you are cruising you shouldnt see a constant ratio, it should be ranging lean and rich, meaning it should be running in closed loop where it monitors air coming into the engine with the afm and the exhaust going out with the o2 sensor and adjusts pulse width accordingly to keep it at an average of 14.7. Under acceleration it should be in closed loop and run a the programmed pulse width, on my car under acceleration i run 13.0 +/- .5. I had a similar issue as you and it ended up being the tps as the engine looks at that to determine when to be in closed loop or open loop. Also check you o2 sensor on both your car and your wideband and your fuel pressure regulator.

1991 E30 M42 · « **Reply #4 on:** April 03, 2009, 12:12:23 AM »

Quote from: tjts1;69306

You can safely up the injectors to 22 or 24# injectors.

You can but after a while the ecu will adjust, I have found out after modifications that the ecu on these cars has adaptive learning and will adjust itself over time to certain modifications.

finite · « **Reply #5 on:** April 03, 2009, 02:17:09 AM »

Hi guys,

Thanks for the info.

I have no o2 sensor in the exhaust, the car never had one. I've replaced the ICV to no avail, TPS seems to be good (haven't measured voltage but response to throttle seems ok). On cruise AFR does move about around +/- 1

Just to clarify the running situation:

1. Starts fine and idles OK, is running closed loop on idle
2. When coming to a stop from cruise or acceleration a bit of heel and toe is required to stop it stalling but will then idle ok
3. When cruising AFR is 15.5 +/- 1
4. When lightly accelerating AFR is around 16.5
5. When WOT AFR is around 17.5+
6. Car feels sluggish under acceleration
7. Spark plugs indicate underfuelling
8. Forgot to mention car has cone filter in cold air box and very derestricted exhaust

On the MAF - a friend of mine did fiddle with the adjustment screw, the one that you're not ment too (under the blue plastic anti-tamper plug). Before he did this I was sure the car was running rich but have no lambda readings to prove it. Since fiddling with the screw I now have lambda readings that are lean. Any further fiddling with the screw has made no change. I've ordered another MAF in case this is now fecked.

I've ordered a fuel pressure guage to check that out.

Vaccuum leaks - where should I look for these? I've already removed the mess from under the TB and removed the heater plate so I'm down to one hose from crank case vent to TB, one from main inlet boot to ICV, and one from TB to FPR. I've also replaced all inlet manifold gaskets. I popped the plugs out to see if maybe one plug was different (possibly indicating a cracked manifold) but all are the same.

So to summarise I need to look at:

1. Vacuum leaks although this is looking unlikely
2. Fuel pressure
3. Electrical issue with MAF or TPS

1991 E30 M42 · « **Reply #6 on:** April 03, 2009, 10:22:00 AM »

the mess under the intake really isnt vacuum related, spray some carb cleaner in between the manifold halves, in between the tb and manifold, and in between the manifold and the head. just listen to any changes in the way the engine is running when you spray the carb cleaner. check the tps it has 2 ranges, one that increases impedance as the tb opens, and one that decreases impedance as the tb opens, i dont remember which pin is which but it wasnt hard to figure out. Fuel pressure is a big thing, once you get your gauge, install it inline on the supply line, the one that goes to the rear of the rail. Fuel pressure should be 3 bar, if it is not don't automatically assume it is the fpr as it could be the pump, or as Jimmy found out, the plastic piece that goes in between the pump and the hard line.

finite · « **Reply #7 on:** April 03, 2009, 11:22:06 AM »

Thanks for the info again

My fuel system looks like this: http://www.realoem.com/bmw/showparts.do?model=AF92&mospid=47256&btnr=16_0043&hg=16&fg=05

What's the plastic part between the pump and the hardline you are referring too?

I'm going to pop off my inlet manifold halves and throttle body tonight and check for evidence of leaks, they have only been on for about 100 miles so should come off with little issue. I was think about this earlier, when I put the top half of the inlet manifold on I trapped an inspection lamp between the manifold and the rocker cover... the lamp crushed and i didn't notice (stupid I know) until after then pulled it out. I suppose this could have vaused the top half of the inlet manifold not to have gone on correctly.

I've also ordered today (as time is running short):

1. New MAF
2. New ICV
3. New FSE adjustable FPR for the m42 rail

If the fuel pump turns out to be out to be no good are there any recommended replacement pumps that I can use rather than standard as getting hold of a standard one looks to be a real problem. I can order a Walboro 255lph pump tonight and have it by Tuesday.

finite · « **Reply #8 on:** April 03, 2009, 06:59:36 PM »

Quick update on this evening's adventures.

Removed inlet elbow, TB, upper inlet manifold and injector rail. Inspected all seals and hoses and noticed the following:

1. Slight evidence of oil vapour seapage around bottom of TB gasket.
2. Pinprick hole in vacuum hose to FPR

So!

Replaced the FPR vacuum hose - doh!
Refitted the TB using some instant gasket on either side of the metal gasket and went for a drive.

First impression better, could see hunting for 14 AFR when idling, but idling at 1100 rpm. So applied some throttle and as soon as cruise was achieved she was still showing 15.5, slight acceleration 16.5, and WOT 17.5.

*sigh*

So by this point it's 01:30 and I don't know whether I'm thinking about fueling problems or little green men, put the car back in the garage and popped the bonnet to disconnect the battery (figured an ecu reset wouldnt go amis)... so what hits me... the TPS isn't connected...

DOH!

So now too late to take noisy track car out for a test, will let you know what happens in the morning!

finite · « **Reply #9 on:** April 04, 2009, 07:11:01 AM »

Corrected my mistake and reconnect the TPS. Went for a drive and the symptoms are still the same.

*sigh*

What next?

1991 E30 M42 · « **Reply #10 on:** April 04, 2009, 12:50:15 PM »

did you check the tps to see if it even works properly?

finite · « **Reply #11 on:** April 05, 2009, 03:10:10 PM »

Tested the TPS as per the Bently manual and it's showing all the correct readings. I'm going to get the car over to the dealer for the codes to be read (assuming they will just read my codes not want to fix the problem and chrage bmw prices!)

One thing I've noticed is that for about the first minute of driving you get the lambda reading's you'd expect, 14.7 ish on cruise, down to about 12 for a bit of throttle on and 10 on WOT (thought I'd test to see, bad when cold I know!). After a minute or two though it's back to the lean symptoms as described in previous posts.

HaNasich · « **Reply #12 on:** April 06, 2009, 07:57:06 AM »

i've measured my M42 and it showed pretty same figures when cruising.
i cant remember where - but i read somewhere that M42 runs lean by default, and on certain situation air/fuel can get very lean and the engine "knows" how to deal with it.
that thread had figures numbers and graphs from the guy who wrote that. i will try later on to see if i can find it - could be on an Australian M42 site or on R3V / E30Tech

Ron.

finite · « **Reply #13 on:** April 06, 2009, 03:49:13 PM »

I'd be really supprised if the AFR reading's I'm getting are OK.

Put an inline fuel pressure guage in tonight, show the following pattern:

Ignition on around 1 bar
Crank builds up to 3 bar after two to three cranks
Showing just under three bar pressure on idle
Slight dip on revving but back to holding steady at just under three bar almost immediately

Sounds pretty good to me... am I right in assuming this means FPR and pump are ok?

... therefore the problem is most likely electrical?

finite · « **Reply #14 on:** April 07, 2009, 05:50:43 PM »

Just got the FSE FPR all fitted, made a nice little bracket to mount it off the upper inlet manfold, all fits nice, one thing I'd say is get about a meter of fuel hose as they don't really give you enough, i've put a joiner in (that's supplied) for the minute. Running it at 3.2 bar static pressure seems to have cured my running issues! Still a little lean on cruise around 15.2 but hits 14.7 on light acceleration increasing to around 12 as acceleration increases to around 10.5 on WOT.

Couple of pointers if you're going to fit a FSE FPR - upper inlet manifold has to come off, fuel rail off. When you put the new adapter in place of the old FPR make sure you grease up the seals. No sensible route for mounting FPR on bulkhead so ignore that in the instructions. Oh and make sure you get the right kit

BOSCH fuel pump on the other hand is not so much fun! Wrong blody pump!

But all in all a good evening! My guess is that although the pressure guage was showing 3 bar static pressure at idle or non-load revving that once it got hot from road driving something in the FPR was giving way and not holding the correct pressure.