Hey guys so very recently one morning I started the car and noticed it took just a split second longer to start and already knew something was wrong. It had a rough idle, decided to drive it and it NEVER stalled even though it sounded like it wanted to but at lower RPM it definitely had no power at all but at higher speeds drove and sounded like normal. It’s a 1991 318i m42, i was reading a thread and someone had the exact issues i had and someone gave him a thread to read but something was wrong with the link so i couldn’t read it, though on that thread some suggested o2 sensor. I was wondering if it’s possible to unplug an o2 sensor to see if there’s a change in how it runs ?
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O2 is below the empty battery tray, round connector, which can be unplugged.
TPS seems a more likely fault. -
Well I do know the TPS does somewhat work because it’s needed to be able to do a stomp test, without it working I wouldn’t get codes. How could I test my TPS? Tmro i’ll unplug my o2 sensor just to see too.Comment
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Hey so I unplugged my o2 and it idle way better, could it have been the issue ? with my MAF unplugged it’ll idle high. I believe my TPS is fine.Originally posted by roguetoaster View PostComment
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Originally posted by ScottySbk View Post
Replace the 02 sensor!'91 318i (Alpine) - daily
'88 325ix (Zinno) - drive here and there!Comment
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yeah it’s not my o2 sensors, i’m left with checking fuel pressure i suppose, just wanna know how could either low or high fuel pressure cause it only to occur normally when i first start it up. Then after a 10 minute drive it wants to idle and have its power somewhat back.Originally posted by fresh_TD View PostComment
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it runs poorly when cold i suppose, i drive it about 10 minutes and it’ll idle great and i won’t have power loss anymore, also checked fuel pressure and when idling it sits just below 29 psi which is obviously low, what could i do to tell me whether its FPR or restricted lines related ? or even pump related i guess, pump was replaced months ago.Originally posted by bmwman91 View PostComment
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If you were reading 29PSI at idle, that is normal. The 3BAR / 45PSI rating is the pressure that the fuel is at relative to the intake manifold, and when idling it is pretty close to a vacuum, so you would only expect to see ~30PSI relative to the atmosphere.
The only thing that really comes to mind is that maybe the coolant temp sensor is dying. Try measuring the resistance across its terminals when it is cold to see what the value is. Also pull back the boot on the plug for it and make sure that the wires are not damaged where they are crimped into the terminals.Comment
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I replaced the coolant temp sensor already and it acts the same :| dang ok tho, yeah i clamped the return line and got no pressure increase and also pulled the vacuum line to the FPR and got no increase. When idling and revving is there suppose to be an increase ? cause it did not move a bit when revving at idle. Honestly lost at this point.Originally posted by bmwman91 View PostComment
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Ok, yeah there is definitely a fuel supply issue in that case. Clamping the return line should increase the pressure for sure, and the only thing that I can think of which would cause pressure to not increase is a dying fuel pump. Even a plugged fuel filter should register some increase with the return clamped.
Your symptoms make a little more sense now, I think. When cold the O2 sensor is ignored so the car would run pretty lean with low pressure. Once warmed up the O2 sensor can detect the lean condition and the ECU will increase injector pulse widths to compensate.Comment
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yeah just now i drove it and it’s popping like crazy before 2k rpm then once it’s past that it’ll drive and idle ok but once the car is parked afterward and idles great it’ll throw a lean/rich code and when at WOT it’ll throw it but go away when i let off. Suppose i’ll pick up a multimeter, how could i test the pump ? never used a multimeter before, can’t be too hard tho. i know my filter isn’t clogged because i checked it a few days ago, it’s a URO fuel pump.Comment
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It would not be unheard of for the pump to be dying even if it is relatively new since URO is one of the "bargain" brands. My experience has been that they are fine for plastic and non-mechanical parts, but I am not really confident in their electrical parts.
As far as the pump, there is not much that you can check with a multimeter, but here are a couple of things.
1) Unplug it and measure the resistance across the terminals. It should be approximately zero Ohms, or whatever the resistance of the test leads is (usually <0.5 ohms).
2) Plug it back in, peel back the boot on the connector and measure the voltage across the terminals when it is running. You should be reading very close to the battery voltage (>13V if the car is running). If you are reading significantly less than the battery voltage, then there is a problem with the wiring or the fuel pump relay.
Being that it sounds like you are having a fuel delivery problem and the engine is running lean, I would seriously avoid driving it hard at all as extreme lean running is dangerous for the engine. The M42 is really tolerant of lean burns in that it won't start pinging until you are dangerously lean (if at all), but running the throttle hard is definitely not doing anything good for combustion chamber temperatures.Comment
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Ok thanks, i’ll do that tonight when i’m home. Also what about any possible debri in the tank messing with the pump ?I also swapped the relay with the 02 sensor relay and no change. if it ain’t the pump i’ll be absolutely fucking lost. It was the last on my list because i replaced it.Originally posted by bmwman91 View PostComment
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Alright last thing man, ohms and voltage were good but it had a fuel drip right at the fuel filter. This drip happened at both ends of it. It’s happened before but it’s very inconsistent sometimes happening at both ends, sometimes at one and sometimes never. im gonna replace them but i can’t imagine how this would cause weak acceleration normally when cold or after sitting for some odd hours and then drive fine and NOT leak after a few minutes. Suppose it could be messing with pressure as well right ? My MAIN question is how it causes the issue at a specific time rather then all the time or more inconsistently. Thanks for all the help, gonna replace them tmro and see how it goesOriginally posted by bmwman91 View PostComment
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