TPS, Vac Leak??? Stumped!

Z3forme

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Sep 10, 2020
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Hello Everybody,

2001 Z3 3.0i with the M54B30 here.
I'm having an issue I've been trying to track down for a good while now. One day I noticed that letting off the gas produced a popping noise and my car smelled like it was running lean (caustic smell). Since it had been 10 years since having done the CCV, I figured I may as well do some preventative maintenance on the way. Decided to change the spark plugs, valve cover gasket, and CCV. I cleaned the MAF, ICV, and TB along the way as well and shipped injectors off to be cleaned. Upon putting her back together again I found that she was still exhibiting very similar symptoms. I'm experiencing a lumpy idle (misfire perhaps every 5 or 6 seconds or so) and when I go to accelerate I get a hesitation and some rapid misfiring (temporary lean condition from what I can gather?) before she takes off. I've double checked that all the boots, CCV hoses and whatnot are in place and ensured that my DISA valve wasn't leaking air around the gasket.
I've plugged my OBDII reader in and viewed some live data to look for oddities. Strangely, my reader gives percentage values for long and short term fuel trim for each bank as well as for each O2 sensor (as well as their voltages). My post-cat O2 sensor's STFTs will read and stay at 99.2% and their voltages like to stay high ~0.7V. This points towards running very lean as well (unless I'm mistaken). At idle my MAF reads ~4.3 g/s with ~20-40 when driving, which makes me think it's probably OK. One odd thing is that my throttle position sensor seems to jump around from anywhere between 0% to 13% when cruising (I thought it was supposed to be %13 completely closed?). I removed my throttle body and checked some resistances on the pins. I read 1383 to 500 ohms across pins 1 and 2 from closed to open, continuous OL across 1 and 3 (read this was suposed to be 1-5kOhm?), continuous 1720Ohms across 1 and 4, and 727-1330Ohms across 1 and 6 from closed to open. I've read that there are two potentiometers that move inverse of eachother and the readouts make sense. Only that pin 1 and 3 has me thinking something may be wrong with the TB.
What do you guys think? Am I just chasing some huge vacuum leak (the M54 bane) that I just can't find for some reason? Is it the throttle position sensor? I had perhaps thought it was an O2 sensor issue at one point, but that the problem persists while running in open loop makes me think otherwise. I had thought it might be my fuel pump, but fuel pressure held at a continuous (albeit rapidly vibrating) 51psi and didn't drop when revving.

Any help is appreciated as I let her sit for 3 months while doing the aforementioned maintenance (very busy school schedule) and was disheartened by the problem not being resolved afterwards. Not to mention the MAF, TB, and Fuel Pump plus Filter are all somewhat pricey parts (Discounting the MAF if I buy the 00 Hyundai Sonata one as it's the same) that I do not want to change them without certainty. I'm eager to get my precious Z back on the road running right!
 

Z3forme

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Sep 10, 2020
Messages
10
I forgot to mention, there is no check engine light! However, when I read the codes I get P1619 ( Map Cooling Thermostat Control Circuit Signal Low) which I suspect may be damaged wiring or a bad ECT sensor in the thermostat housing. Checking the ECT temp I get a consistent reading of about 90C. Wouldn't think so, but could said code cause a problem like this?
 

t-tony

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You can double check temperatures using in infra red thermometer, readily available and not expensive. Useful for other jobs too.

Tony.
 

Nodzed

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@colb or @mrscalex may be able to offer some advice here. ..................... Guys?
 

colb

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High fuel trims is a good indication of air leaks being present best check by smoke testing it that will find any leaks you cant find visually. Once certain you have no air leaks and it's still not right then turn attention to the Maf, when they go bad and misreport what's passing through them a symptom is the engine being held back and not performing as it should when accelerating. MAFS don't always set codes when they go wrong and expensive to replace, cleaning them didn't work for me replacement with oem part solved my issue immediately.
Where are you, if near me post lockdown could smoke test it for you. Can't comment on the Map sensor not had any problems with that so far
 

Woodsta888

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Exmouth, Devon
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2.0
Not saying it's the answer but worth a check. I had an intermittent lumpy idle and misfire, sent it to the indy, £400 later and a new MAF it was the same next time I started it at home. Check the hoses to teh charcoal filter and the solenoid to that filter as they break down. Cost £10 for 2nd hand solenoid. Worth a check.

DO you get a big rush of air when removing the fuel cap after running engine?
 

colb

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When checking for air leaks, don't just concentrate on looking for these on the rubber and plastic pipes of the intake and vaccum hoses, also check across the entire engine, smoke will be your best bet for finding any obscure places that have leaks. Case study I had on a members 2.2 at first revealed badly fitted lower intake bellows to the throttle body and a suspect sucking jet valve adjacent to the throttle body. Replaced the sucking jet valve and correctly fitted the intake bellows to the throttle body to eliminate the air leaks that smoke had found there. Retested with smoke and with that side of the engine sealed it found a further leak comming from No4 spark plug well on the cam cover. Changed the cam cover gaskets which included the centre gaskets around the plug wells. All back together and retest with smoke, alas smoke still seen from No 4 plug well area. Strip down and close exam of cam cover found the plastic had gone brittle and developed cracks and on poking it it just started to crumble. A good used cover was obtained and put on the car which following a smoke test proved the vaccum system was now sealed. Transformed the car and returned fuel trims to normal. One other point to check is the O ring in the dipstick tube if you have a dipstick, these rings fail and cause an air leak. Smoke will find that if its a problem.
 

Z3forme

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Sep 10, 2020
Messages
10
The reader I have will display some live data, but I can't get it to record so I'm unfortunately stuck with having a passenger monitor the values and record them as I drive. So far I've recorded the following while driving at 60mph. STFT Bank 2 varies between 3.1% and 7.8%. LTFT Bank 2 sits continuously at 0%. STFT Bank 1 varies from between -1.6% to 9.4%. LTFT Bank 1 varies between 3.1% to 4.7%.

From what I've been reading these fuel trims are not crazy high and I'm thinking perhaps a small vacuum leak somewhere? I think I was a bit confused earlier and believe I have made sense of the post-cat values as the O2 sensors there are supposed to read lean if I'm correct (my scanner seems to convert O2 sensor voltage to STFT %, so a steady high reading would indicate my cats are working [anybody please correct me if I'm wrong]). Always feel like ruling out cats is dodging a bullet. *knock on wood*.

Tony, I'll definitely be borrowing an IR thermometer from my lab to verify that the ECT does align with the thermostat. I'm assuming I'll just examine the housing and nearby exposed block with the expectation of being a few degrees below.

Colb, we are unfortunately about half a world apart as I live in Texas. Thank you for the offer though! I've so far examined the SJP, CCV and dipstick o-ring, VCG, and boots for leaks and everything appears in one piece. I'm thinking you're right and a smoke test may be the wisest thing to do. I'll definitely have to see if I can rent or source a smoke machine (I've definitely needed and debated it on quite the many occasions and it really appears invaluable with the M54s).

Woodsta, I will definitely be examining the charcoal filter hoses and solenoid as well as checking the fuel cap (I can't say I've ever noticed any rush of air when going to get gas, but it's certainly worth checking). Thank you for the advice! I would have never thought to look there as a potential cause.

I'm honestly considering seeing if a local junkyard has a BMW with an m54b30 laying around that I could get the MAF off of (they must with how many models this engine was used in, right?)

Thank you all for the help! This problem has really had me guessing at a host of potential (scary/expensive) problems from Fuel Pump to MAF to TPS to O2 sensors ect. I've never wanted to see a check engine light more than I do right now!
 

t-tony

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Also try a spray of electrical contact cleaner on the connectors. Can work wonders.

Tony.
 

colb

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I made a home made smoke machine using a new empty 5l paint can and resistance wire plus bits I had in the garage. Very similar to this on youtube
View: https://www.youtube.com/watch?v=Np_duzsaZxI&t=301s

I use a bycycle pump to produce the air flow to blow the smoke out, it works a treat using Baby oil. Other versions on youtube use glass pickle jars and a mounted soldering iron to produce the heating to produce the smoke. All versions on youtube if you search smoke machine for cars.
 

Z3forme

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Sep 10, 2020
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10
Ahh, a soldering iron as the heat source! Of course! That's much better than buying or renting a fog machine. I'll be building one as soon as I have the chance.
 

colb

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Just remembered another place to look for air leaks are the rubber blanking plugs that are found hidden at the rear underside edge of the inlet manifold. These have a habit of perishing and splitting and eventually falling off which opens up the vaccum, easy to miss these if you dont know they should be there. Smoke will of course find them if they are the issue.
 

NZ00Z3

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The below is what I use when diagnosing a problem with Z3's or E46's.

Did you remember to fit the rubber cap on the nipple located in the middle of the CCV regulator? Sometimes it is missed on M54 engines and causes a hard to find vacuum leak.

Do you have a vacuum Leak?

The proportion of unmetered air from a vacuum leak to metered air is higher at idle and lower at revs. Do this test.

With a hot engine at idle, check the 4 fuel trims, 2 short-term and 2 long-term. If any are > 8% then you likely have a vacuum leak. Rev the engine to 3,000 rpm for 30 seconds and watch the short-term fuel trims. If they reduce significantly, might even go negative, then it is confirmed that you have a vacuum leak. Smoke test the engine to find the leaks.

If the short-term fuel trims do not reduce significantly, then it’s something else.


Do you have a fuel supply problem?

The fuel supply system has to deliver 2 key functions:

1) Fuel pressure. It should be 50 PSI at the fuel rail Schrader test valve with the key at position 2 and the engine not running. The fuel pressure regulator is part of the fuel filter. It could also be the fuel pump.

2) Fuel volume. If your fuel trims are +/- 5% at idle, but get worse, > 8% when at highway speeds, then you could have a fuel volume problem caused by a blocked fuel filter or a failing fuel pump.


Do you have a faulty MAF?

The MAF should be drawing around 3.5 g/s at hot idle.

Make sure that the MAF is marked BMW or Siemens/VDO. The cheap Chinese knock-off MAF's or the Hyundai part number MAF's don't work well on the Z3’s or E46's.

Many people unplug the MAF and discover that the rough idle or lean condition disappears. They conclude that the MAF is faulty. This is not always the case. When you unplug the MAF the Engine computer (DME) reverts to using fuelling tables already programmed into it. These fuelling tables are richer. This richness often compensates for a lean running condition caused by vacuum leaks. So, confirm that you don't have any vacuum leaks before unplugging the MAF. If you then unplug the MAF and it does improve the lean condition, changing the MAF with a good quality one is your next step.
 

Z3forme

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Sep 10, 2020
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Hello Everybody,

Been a while and I've been incredibly busy, but I finally discovered and fixed the issue a few weeks ago! Posting the rundown in hopes that this info may help others one day.

So my issue started with a come and go rough idle that felt like misfiring and progressively got worse without throwing any codes. As is with these engines, especially ones approaching 200,000 miles, the first thing to do was searching out vacuum leaks. Thanks to the help of you members I managed to find deteriorated cap plugs and a degraded vacuum line near the non-return valve in the secondary air system (Had done the lines leading from the secondary air pump to the electronic valve, but hadn't gotten the ones behind the valve!). While these vacuum leaks did need to be addressed, they didn't resolve the issue. I had just done the CCV (figured after 100,000 miles and 10 years it surely needed replacing or at least would very soon) and taken care of all the vacuum leaks according to the smoke tester I made. After borrowing a scanner from a friend and reading the MAF sensor draw at idle (about 4.5g/s before the issue was solved, haven't checked after) and concluding it was doing its job I tested the potentiometers on the throttle body and both inversely moved from their max to min resistances so I concluded that the throttle body was fine. I had also recently done the valve cover gasket as it had a small leak and decided to do the spark plugs while I was in there. I figured at high mileage fuel injectors should have been cleaned too so I sent them to InjectorRX in Houston and am happy with the data and results they provided. The worst injector was approximately 12% clogged. Although the engine compensates with pulse width increases, I like to know that the injectors are now clean, balanced, and free of leaks. Tested the resistances on the ignition coils and they were all the same so I assumed they were good as well. Checked the fuel pressure and it read a steady 51 psi as it should so I figured the fuel pump was good (had done the fuel filter perhaps 20,000 miles ago).
At this point I was feeling quite frustrated as it should have been running better than I had ever seen it run (the car was 10 years old and somewhat neglected when I got it 10 years ago). Nothing more frustrating than the engine idle and drive-ability slowly perishing without throwing any codes!
On a whim I decided that there was a nonzero chance that my 5 month old battery was bad and maybe that was why no codes were being thrown. Had my battery tested and surely enough it came back bad! Had it replaced under warranty and thought "This must be it!!!". Turned the car on and surely enough it still ran terribly......BUT the check engine light came on!!! Read the codes and got a misfire on cylinder number 5. Swapped the ignition coils between cylinders 5 and 1 and surely enough the misfire followed. I had found the issue! As I refuse to replace any engine critical components with anything but OEM, I look for a new ignition coil and find them to be somewhat out of the budget range of a broke college student. A quick trip to the local BYOT junkyard and I manage to find an 04 328i (Not a lot of BMWs there). The ignition coils it had were Bosch as I believe they switched from Bremi to Bosch in 2003 or 2004. There was a 1994 730i with what appeared to be the same Bremi coils I had, but I couldn't convince myself to grab a 27 year old Bremi coil over a 17 year old Bosch coil. Purchased the Bosch coil for a whopping $12! For those of you wondering, the Bosch coils work in place of the Bremis with no problem. I installed the coil and it FIXED THE PROBLEM!!!!!! Getting to replace/clean/repair so many things on the engine and not getting to feel the benefit of any of them before putting that last puzzle piece together was an experience I can't explain with words. This thing went from months of being in a borderline unusable "I feel like I'm dying" condition to suddenly feeling like it warped back in time to the day it was made. I can't testify how it ran when it was made as I was only 5 years old, but the way it runs now puts the way it felt 10 years ago and anything since to shame.
Long story short, ignition coils can be bad even if their resistance reads correct and a (somewhat) bad battery can somehow have no symptoms aside from preventing a check engine light from coming on (unless it's to blame for the ignition coil going bad.....seems unlikely though).

All and all, after 10 years of ownership and 100,000 driven miles I still get excited every single time I drive my Z3. It still gets regular complements from strangers and far too many people ask if, or assume that, it's brand new (have they not looked at cars for the last 26 years?) so it doesn't seem to have gone out of style by any means 😉, not that I ever get tired of looking at it. Anyhow, I hope this post helps someone with their cherished ultimate driving machine one day!

Happy Driving!
 

IanA

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Super job!!! Always good to hear of a problem solved. Coil packs do get a bad press. I had to replace the 4-in-1 coil pack on a GM engine twice in quick succession. Occasional misfiring which cleared with a restart, but then brisk acceleration brought it on again. Mechanic's comment: "They're known for it".
 

Althulas

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Nice detective work methodically done. Nice to get to a result especially when being thrown a ref herring. Well done and one to keep in mind.
 
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