Tell me if these readings make any sense

[BennyM]

Still trying to figure out my M50TU's deceleration to idle hesitation. My current theory is that it only happens when the DME tries to run in closed loop mode. I have all new sensors on the way, but help me make sense of the se readings before I go broke throwing new parts at this thing:


TEMP SENSOR measurements AMBIENT [acceptable range 2100-2900]
Coolant Temp (black) 5,570 Ohms
Engine Temp (blue) 2,006 Ohms
Intake Manifold Temp 1,768 Ohms

TEMP SENSOR measurements HOT (10 min after running) [Acceptable range 210-400]
Coolant Temp (black) 667ohms
Engine Temp (blue) 332 Ohms
Intake Manifold Temp 1,598 Ohms

Diagnosis:
The coolant temp sensor seems to be reading colder than it should. Ambient temperature was 80F, but it was reading more like 30F, and even when hot the coolant temp seemed lower than expected. This would also make the engine run richer than it should, especially on a cold start. Right?

FUEL PRESSURE (in PSI)

Idle 45
High RPM 45
Engine Decel from Rev 49 down to 40, back to 45
Vac Hose Off 52
Engine Off 40 (drops to 20 in a half hour, drops to 0 by the next day)

The FPR seems to be doing its job, but every measurement is about 10psi lower than spec. I've got a leak. But I don't see or smell any. All plugs are identically black from running rich, so I don't think the injectors are leaking (unless all six are leaking).

O2 SENSOR
Heater Resistance, but no voltage from computer to activate
Sensor Stayed around .18v, fluctuated only as high as 0.4v

DME may not turn on heater if it thinks the sensor is bad? Why doesn't it want to heat the 02 sensor?

Thoughts?

[Blitzkrieg Bob]

DME may not turn on heater if it thinks the sensor is bad? Why doesn't it want to heat the 02 sensor?

Thoughts?

Relay? But that would only happen when the car is cold. Once you warm up, you're fine.

Check out all the hoses under the manifold.

I had a problem a while back with my touring and it turned out to be a hose between the ICV and the manifold. Just a crack, but it would throw error codes for the 02 sensor.

[Ross]

Pretty sure your M50 should have a3.5 bar fuel pressure reg., it's marked on it. 45 psi is too low if so.
Your other indications are the pressure bleeding down and apparent rich running, also things attributable to a FPR.
Check pump pressure to be sure it is capable of more, a good one will be ~70 PSI at the outlet. The bleeding down points at the regulator though.
Low pressure results in crappy atomization from the injector causing your sooty plugs.
Before throwing your wallet at a bunch of sensors that you've ostensibly tested and believe good I'd be swapping in a fresh FPR.

[BennyM]

Thanks for the feedback. I've posted a few times now covering my progress on this engine issue, but you probably missed those. The first thing I did was replace all the hoses and seals. I had a bunch of leaks at the ICV. After that I checked again for leaks with carb cleaner and found none.

The O2 relay was bad, but then I put a new one in and the computer still wouldn't activate it. I tested that the computer could send a triggering voltage for the relay with ignition on, and it would. But as soon as I put the relay in, the computer would choose to not send any power to it. I tried two more relays with the same results. Totally weird.

Relay? But that would only happen when the car is cold. Once you warm up, you're fine.

Check out all the hoses under the manifold.

I had a problem a while back with my touring and it turned out to be a hose between the ICV and the manifold. Just a crack, but it would throw error codes for the 02 sensor.

[whiskychaser]

Pretty sure your M50 should have a3.5 bar fuel pressure reg., it's marked on it. 45 psi is too low if so.
Your other indications are the pressure bleeding down and apparent rich running, also things attributable to a FPR.
Check pump pressure to be sure it is capable of more, a good one will be ~70 PSI at the outlet. The bleeding down points at the regulator though.
Low pressure results in crappy atomization from the injector causing your sooty plugs.
Before throwing your wallet at a bunch of sensors that you've ostensibly tested and believe good I'd be swapping in a fresh FPR.

3.5 bar is 51 psi. You have 52 with the vac hose off which means its just about bang on. With the engine running you are getting 45psi with it on. You knock off about half a psi of fuel pressure for every inch of HG vacuum the engine creates assuming you are at sea level. Ostensibly you have got about 14 in HG inlet vacuum. Thats a bit low. So maybe a worn engine or you just live up a big hill? Either way I dont recall you saying it wouldnt start?

[BennyM]

Hey Ross. Thanks for the insight. I just checked the fuel pump with the engine off. It ran at 54psi, a little higher than spec. And when I pinched off the return line and ran the pump momentarily, it instantly jumped to 70 psi. I kept the return line clamped and let the system sit for an hour. After an hour, the pressure dropped about 2psi. I unclamped the return and within 5 min, it had dropped 7 psi. Definitely seems like the FPR is leaking. Good call.

I do think a couple of the sensors were giving wacky readings, so I don't feel like I'm wasting money on those.

Thanks.
BennyM

[BennyM]

The engine starts right up every time, even if there's no pressure in the fuel line (just takes a couple more cranks). Delivery and spark seem solid. I'm about 800 ft above sea level.

And I know the bently says that pressure should be a little lower than spec when the engine is running. Does "a little lower" mean 5 psi? I'm sure the vacuum isn't as strong as a new engine, but I don't follow your calculation. Are you saying the operating pressure should be even lower? Like 43psi?

I'm not concerned about the system pressure when it's running as much as I'm concerned about the pressure dissipating rapidly when it's not running. Please correct me if I'm wrong, but my understanding is that with everything off, the system should stay pressurized for a long ass time. Within a half hour, mine'll drop 15 psi. The seems too fast. Right?

3.5 bar is 51 psi. You have 52 with the vac hose off which means its just about bang on. With the engine running you are getting 45psi with it on. You knock off about half a psi of fuel pressure for every inch of HG vacuum the engine creates assuming you are at sea level. Ostensibly you have got about 14 in HG inlet vacuum. Thats a bit low. So maybe a worn engine or you just live up a big hill? Either way I dont recall you saying it wouldnt start?

[Ross]

Pressure should be regulated to 3.5/52psi with the vac hose connected. Yours is a bit low. Given the rather subtle nature of your symptoms and the leakdown after shut off I'm confident the FPR is at fault.
Are your cold starts rough???

[ryan roopnarine]

if you look at bruno's wiki at bmwe34.net, there are instructions as to how to install the retroactive fuel pump check valve kit. i got mine from the dealer about 3-4 years ago for under $10, might be higher now. i'm not saying that this is what is causing your problem, but you might want to do it at some point (even if that means on a new fuel pump).

that being said....where did you measure your fuel pressure, at the rail, or at the pump? ymmv, but if your fuel filter is fresh, there's something i like to do that lubes fuel pumps (tends to quiet them down a good deal for a long time), USUALLY bumps the pressure up at least 1 or 2 psi if the unit is marginal or poor, unsticks sticky fuel floats, and strips the shiat out of carbon on your pistons. do this at your own risk, as i don't know what state your equipment is in. pour a quart of marvel mystery into 10 (preferably 5) gallons of fuel and run till your light comes on. you might want to wait until an oil change to do this, though.

ps...take the end of the vacuum hose off the fpr after running and see if it reeks of pure gasoline. that'll end the speculation quick.

[BennyM]

Hey Ryan. That's some good info. I will probably try that retro check valve in the future. I measured at the inlet of the fuel rail. I've tried other fuel additives before an oil change like that, but never tried the Marvel Mystery stuff. I've been considering a soaking of BG 44K once I get the engine running a little more stable. Seemed to help my other M50 years ago.

I have a new FPR and new sensors on the way now. I'll let you all know how it runs once I put them in.