SOT - e30 325i fuel supply cuts out at ~40mph

MicahO · Post by **MicahO** » Aug 05, 2012 12:21 PM

I've got my brother-in-law's e30 325ic here. It's a bit of a basket case, but the timing belt is relatively new. The thing would not run nicely at all - though that was all just simple stuff. Having replaced the intake boot and some vacuum lines, cleaned up some grounds and cleaned out the throttle body, ICV and AFM, the car now runs pretty nicely. It still needs more attention, but the car now runs and idles fairly smoothly.

But it will not propel itself above about 40mph. I can run first gear to redline with nice power. Throttle response is solid. Second gear pulls nicely to about 40mph, as does third gear and fourth. But at ~40mph (a bit higher, maybe 42-45?) the car just stops delivering fuel. If I coast down a hill and let gravity take the car over ~40, the engine shuts down.

So it gets fuel all the way to redline below 40mph, but fuel cuts off just above 40.

What system on these cars would deny fuel supply based on road speed?

TSMacNeil · Post by **TSMacNeil** » Aug 05, 2012 1:06 PM

I dont have any books in front of me, but doesn't Motronic cut the injectors over xyz engine speed (RPM) and TPS idle?

Maybe a TPS issue.
http://www.mye28.com/viewtopic.php?t=91137

"Deceleration cut- off"

MicahO · Post by **MicahO** » Aug 05, 2012 2:13 PM

TSMacNeil wrote:I dont have any books in front of me, but doesn't Motronic cut the injectors over xyz engine speed (RPM) and TPS idle?

Maybe a TPS issue.
http://www.mye28.com/viewtopic.php?t=91137

"Deceleration cut- off"

The deceleration cutoff seems to be an ASC function, and there is no asc on this car.

But the TPS could be a possibility? I have not tested the unit that is in the car. The car does return to idle now after the other basic work that was done. But these TPS's are upside down under the throttle body and have a tendency to fill with oil. The throttle body WAS admittedly oily, as was the ICV. Perhaps it is goobered up.

Still, please keep in mind that this is not in any way engine speed related. I can run first gear hard out to redline. I can run second gear up into the upper reaches of the tacho, until it hits 40mph. But coasting downhill, in neutral, engine at idle, as gravity takes the car past ~42mph, the engine just turns off - bingo fuel.

Engine speed I've got. Road speed I do not. Confusing.

TSMacNeil · Post by **TSMacNeil** » Aug 05, 2012 2:21 PM

Hmmm
Perplexing.
I dont think the decel cutoff is ASC related, but 1000-1200 rpm and idle signal. Id check the tps for intermittants.

"Deceleration fuel cut-off
When the engine speed is above 1000 . . 1200 rpm with the throttle closed, Motronic cuts off injector operation and retards the ignition timing as an economy aid. As the engine speed falls below the threshold value, injection is re-instated and the timing advances. The point at which injection is re-instated depends upon engine temperature and the drop in rpm. "

If its truly speedo/sensor related I'm at a loss too.

MicahO · Post by **MicahO** » Aug 06, 2012 1:26 PM

TSMacNeil wrote:"Deceleration fuel cut-off
When the engine speed is above 1000 . . 1200 rpm with the throttle closed, Motronic cuts off injector operation and retards the ignition timing as an economy aid. As the engine speed falls below the threshold value, injection is re-instated and the timing advances. The point at which injection is re-instated depends upon engine temperature and the drop in rpm. "

Ah yes - that decel cutoff. That's one way that fuel gets cut to all of these engines. Maybe *the* way.

I will say that the dash has issues. The temp gauge jumps all over the place (though watching the hoses and thermostat housing with an IR thermometer show normal temps). So at the very least the ground on that gauge is probably crapped out. Perhaps there are other issues in the dash that somehow crossover and trigger the decel cutoff?

Ugh!

I'll yank it out and see

TSMacNeil · Post by **TSMacNeil** » Aug 06, 2012 1:41 PM

I can't think of anything else that would cause the engine to just stop. I'm wondering if the 42mph speed is just a coincidence, and something is else is really triggering the ECU decel mode...it's the only thing that makes sense.

tn535i · Post by **tn535i** » Aug 06, 2012 1:50 PM

The TPS when in 'idle mode' cuts fuel above 1500 rpm. Just unplug it to see if it's possible, but from your description NO it's not. Even if WOT and idle are somehow crossed you would still accelerate and run fine over 40 if in between idle and WOT.

There is no other rev limiting feature I've ever heard of on an e30 other than idle mode and the ECM based on speed (we've had an 84, 86 and 91). These cars were all before electronic nannies.

Is it remotely possible that you have an OBC with trip computer and somehow the speed alarm is shorting/crossing/killing the ignition or fuel? If you have the OBC with speed alarm have you tried setting it higher (if it is set to 40 mph now)?

Very strange...

ahab · Post by **ahab** » Aug 06, 2012 1:53 PM

I think if you pull the diff sender there will be no way for the car to know road speed and you can eliminate the OBC or ECU intervening. I don't know that the ECU senses road speed but without a diff sender you can focus on it being something more obtuse! If you coast it up to say 50mph, does it drop to idle or shut down completely?

TSMacNeil · Post by **TSMacNeil** » Aug 06, 2012 2:03 PM

ahab wrote:I think if you pull the diff sender there will be no way for the car to know road speed and you can eliminate the OBC or ECU intervening. I don't know that the ECU senses road speed but without a diff sender you can focus on it being something more obtuse! If you coast it up to say 50mph, does it drop to idle or shut down completely?

^^great idea, that's what I'd do next.

MicahO · Post by **MicahO** » Aug 06, 2012 2:11 PM

Coasting up into the mid-40's (i.e. down a hill) causes the engine to shut down - no fuel. That's when then engine is at idle, in neutral, just rolling along. Splat!

Tyler's note -

This OBC does have the speed alarm feature. The OBC is operational (though with dead LED bulbs). Yesterday (in bright light) I checked that the speed alarm was not set, though I did not attempt to set it myself to another value. That's a good idea, long shot or not.

As Tim mentioned, I'm going through in my mind what random other item could be happening around 40mph that causes a sensor or wire or mounting tab to bend out of tolerance and lose signal.

Unplugging the diff sender is definitely the quickest test. I should have done that yesterday morning while I was a) waiting for the battery to recharge, and b) underneath there wrestling new exhaust hangers back onto the car.

ahab · Post by **ahab** » Aug 06, 2012 2:29 PM

FWIW, you may want to see if setting the timer and the speed limiter causes the red square in the button to light up. This would at least confirm positive engagement of each function. Also, there is a beep when you hit the speed limiter. I haven't used it in a while but I'm pretty sure it doesn't cut fuel OFF, just limits delivery. Is this right?

TSMacNeil · Post by **TSMacNeil** » Aug 06, 2012 2:36 PM

ahab wrote:FWIW, you may want to see if setting the timer and the speed limiter causes the red square in the button to light up. This would at least confirm positive engagement of each function. Also, there is a beep when you hit the speed limiter. I haven't used it in a while but I'm pretty sure it doesn't cut fuel OFF, just limits delivery. Is this right?

Maximum speed cut-off
Once the engine speed reaches a pre-determined limit (4 cyl: 6200 ± 40), (6 cyl: 6400 ± 40), Motronic cuts off injector operation so limiting the maximum rpm possible.

Sounds like it cuts all the fuel...could be worth looking at, even though it's rpm not mph...
?

tn535i · Post by **tn535i** » Aug 06, 2012 6:03 PM

ahab wrote:FWIW, you may want to see if setting the timer and the speed limiter causes the red square in the button to light up. This would at least confirm positive engagement of each function. Also, there is a beep when you hit the speed limiter. I haven't used it in a while but I'm pretty sure it doesn't cut fuel OFF, just limits delivery. Is this right?

It should just beep. Unless, some dolt has wired it differently. If it works try setting it to 25 mph and see if it beeps but does nothing to limit power.

This is so very weird... Unplugging the speedo sender is the best idea. Beyond that there is no explanation I can think of other than some weird issue with a connection or harness affected by wind resistance in which case it should let you go a little faster with a good tailwind... or even slower with a headwind. Maybe the primary wire from the coil moves and shorts based on this?? Grasping for ideas??

Does it cut out like it loses fuel or like the ignition is shut off?

MicahO · Post by **MicahO** » Aug 06, 2012 9:46 PM

tn535i wrote: Does it cut out like it loses fuel or like the ignition is shut off?

Excellent question. More descriptiveness, or at least as much as I can muster.

If I slingshot the car past the terminal velocity, it cuts out and above that speed I've got apparently no fuel delivery. In that case, rushing past ~40mph, the cutoff is rather abrupt.

If I coast down a hill, idling, in neutral, the engine basically just shuts down as I pass the terminal velocity.

But if I try to slowly accelerate past that speed, the car will sort of kick, sputter, fart. So when it's *right on* the speed where it dies, it seems to flip back and forth runstoprunstoprunstop. In this case it feels like fuel which is why I've focused on fuel delivery? But I guess it could be either fuel or ignition.

I'm trying to describe something very strange, so I end up sounding a bit 'off.' I'm hoping it's something very simple. I've re-seated and inspected all of the fuses and relays. The relays look old and in some cases toasty on the contacts. But they seem functional. I had some BSA meetings tonight, so no time for tests. I'll see what I can do tomorrow.

I very much appreciate the extra eyes in troubleshooting!

m535is · Post by **m535is** » Aug 06, 2012 10:12 PM

Sounds to me like a vehicle speed input issue. Motronic 1.3 has a vehicle speed input. This input is used for many things, but one is to be a top speed limiter so that the car cannot exceed a given top speed. Potentially if the signal being sent is wrong and the DME sees it as you are going too fast it will shut the motor down to try to limit the top speed of the car. If this is the case you need to figure out why the vehicle speed signal is so off. Also I am assuming this happens regardless of what gear you have the car in.

ahab · Post by **ahab** » Aug 07, 2012 9:44 AM

BTW - stupid side note. I set my road speed warning last night and it simply chimes once and then flashes the red light in the button when you exceed the setpoint. I couldn't remember if there was some actual speed governance, but not surprisingly (and thankfully) that's not the case. I think Rob is on to it with M1.3. Bad ECU?? Does that signal get processed by the cluster and fed to the ECU or does it go in raw?

TSMacNeil · Post by **TSMacNeil** » Aug 07, 2012 9:59 AM

I re-read the Motronic ops list posted in this thread again. From what I read, there are only a few occaisions when Motronic will cut or trim the fuel:

1) "Idle mode" 1000-1200 RPM and TPS idle signal received.
2) Max Eng speed: 6400 +/- 40 RPM
3) AFM flap load
4) Anti-theft function

I'd swap out the ECU first, then swap out the AFM.

( I'd guess the "Max Eng Spd" mode is processed through the tach, which you say may be flaky...so there's a possibility)

MicahO · Post by **MicahO** » Aug 07, 2012 10:31 AM

Digging through the wiring diagrams (looking here at the moment), the speed sensor input seems to go straight to the cluster. Page 1360-7 shows pin 29 for the ECU speed input is not used in that iteration (perhaps it was reserved for future ASC type functions?). Or am I reading that wrong?

This would indicate that the speed signal goes right to the cluster, and the problem probably starts and stops there. Not a bad thing.....

http://www.e30tech.com/forum/showthread.php?t=73541 - Off on a tangent, that's some good stuff. Apparently one can really tweak that signal.

ahab · Post by **ahab** » Aug 07, 2012 10:40 AM

MicahO wrote:http://www.e30tech.com/forum/showthread.php?t=73541

m535is · Post by **m535is** » Aug 08, 2012 2:06 AM

So is the car an '87 or not? If it is then you most likely are not running M1.3, but you are probably running M1.2. Anyway, M1.3 has the vehicle speed input used, M1.2 it is not required. It is not reserved for ASC functions at all. It is used on E24s M30B35s with M1.3 and all later M versions. It is used to be able to do electronic top speed limiters and also to be able in much later versions to be able to tell how much to advance timing etc, based off of how fast the vehicle/engine can accelerate.

ASC functions are all done through ABS wheel speed sensors BTW.

Anyway, the way the signal works is the following. A 5V output is sent by the DME to the cluster. The cluster then has a potentiometer which varies based off of vehicle speed. This is the same signal as used in the cruise control and OBC functions.

One other thing to check. What is the max speed for the convertible top operation and what happens when you exceed that speed? This should be clear in the owner's manual. Not sure if they figured out how to limit the vehicle speed if the top was not up or down, but you never know.

MicahO · Post by **MicahO** » Aug 08, 2012 7:21 AM

Rob - thanks for the clarity on the various functions. Knowing where the signals are going definitely creates a roadmap to where to find the problem. This is an '87 - 5/87 build.

A little tinkering last night. With the max speed set at 32 on the OBC, the OBC makes noise but does not take any other action. With the diff sensor disconnected, the car can go as fast as you make it go, in any gear, no issues. (Well, I wouldn't say *no* issues - once you make it go faster, you can really tell how much of the suspension bushings need replacement, but I digress.....)

I wiped off the sensor (not that it was very dirty) and cleaned the contacts (they looked decent too) and plugged it back in. The phantom top-speed-limiter returned. Letting the car coast above the terminal speed and shut itself down, I paid more attention to what else was going on. The speedo continued to point to the actual speed, but the tach and temp gauges started wildly bucking all over the place as soon as the engine turned off. So with the engine running, the engine speed signal seems to make the tacho give a good read, but with the engine off and a road speed signal coming in, the tacho was all over the place; very jumpy. The temp gauge has been jumpy for a while, but it was REALLY jumpy with the engine off and the car rolling at 45 or so. I think that indicates some significant issues in the cluster - signals crossed or bridged or some other mayhem.

WAG's are about all we've got to go on, but I'm going to try to get a replacement cluster to throw into the car, as this one seems flaky. There are still other issues - many.

tn535i · Post by **tn535i** » Aug 08, 2012 8:48 AM

So maybe the speed input signal is somehow crossing with the tach input and the higher speed input results in an engine speed limit? Sounds like next thing to try would be unplugging the instrument cluster?

Oh but then you won't be able to tell how fast you are going...

m535is · Post by **m535is** » Aug 08, 2012 10:04 AM

Unplugging the cluster might cause another problem. Depending on the programming level, if it really is the vehicle speed input that is causing the problem, I would not be surprised if it also has the "I am connected" signal, and if that is removed causes the DME to go into limp home mode. I still am wondering about how the convertible top could maybe be linked into this. Have you tried the top speed with both the top up and down?

MicahO · Post by **MicahO** » Aug 08, 2012 11:22 AM

Last night's tests were run top-up, though all prior tests were run top-down. There did not seem to be a difference in the results given top-up vs top-down tests.

With the diff sensor unplugged and the car running as fast as I told it to, I was easily getting to 60-65 on the 50mph 2-lane that runs nearby the house. I didn't want to go much faster than that last night. The 2-lane in question runs past the Town Hall/Police Department for my town. After re-connecting the diff sensor, I went back out and tried to sling past the 40-ish cutoff speed. The car mercifully did not let me zoom up to 60+. As I came around the bend at the Town Hall (past the mystery cutoff speed so I was rolling with no power), there was John Law recently set up in a speed trap.... Phew!

TSMacNeil · Post by **TSMacNeil** » Aug 08, 2012 11:26 AM

This is really a weird one.

m535is · Post by **m535is** » Aug 08, 2012 11:30 AM

I say a cluster swap is in order. Nice to have your car save you from a ticket

1st 5er · Post by **1st 5er** » Aug 08, 2012 9:16 PM

Because I got a PM;
'88 January is the info on the 'is' build date.
It had been 5 speed swapped, if that makes any difference, I don't think so on the cluster.
HTH

MicahO · Post by **MicahO** » Aug 12, 2012 4:30 PM

Sherman - thanks for the info on the cluster, and Aaron - thanks for swinging by with the part! Frankly, I'm glad we spent your visit driving different vehicles around and shooting the breeze rather than trying to get the 'vert running right. Aaron was even exposed to 'the routine' with the RetroMom wagon. He was patient enough to ride shotgun while I went round and round the cul-de-sac while my kids bashed around in the backityback of the wagon.

I'm drawing a line in the sand on this car and I'll let my bro-in-law decide what he wants to do. The car now runs a lot better than it did, through a combination of simple repairs and masking of the apparent computer problem. I still tend to think he should move it along and let it be someone else's problem, though I have to admit that I now see the fun in driving the little convertible.

The 'new' cluster, a VDO unit out of an '88, behaved exactly the same as the original Motometer '87 unit that is again reinstalled (except that the temp gauge on the 'new' cluster worked perfectly!). So the problem is apparently not in the cluster. After reinstalling the original cluster, I remembered reading on one threads or another that the green wires on the back of the cluster were the speed-signal outputs. Since the cluster was 'not the problem' then maybe if I disconnected those outputs......VOILA! With no speed signal outputs connected, the car can run as fast as you flog it, and actually moves pretty well. Fun to scoot around in.

Well, sort of fun. There has also been some regression in other areas. It's more obvious than ever that the tires are shot. There is a lot of tread, but there are HUGE areas of sidewall with massive dry-rot on them. A lot of the shimmy and shake on the car is probably from those shitty tires. And there are certainly a bunch of bushings and mounts to swap out. And the starter relay or ignition switch is now dead, so I'm starting the car by turning the key on and then jumping 11-14 on the diagnostic port. Kicks right over, but it's not real graceful

And the reality is that all I did by disconnecting those signal outs is kick the can down the road (isn't that the American way?

). The ECU is probably toast or on it's way to toasted. So the list goes on.....

It's been a real team effort, with good results, and very much appreciated. Thanks guys for all the info and pointers!

m535is · Post by **m535is** » Aug 12, 2012 5:53 PM

Does the cruise still work or did you disconnect all speed outputs from the cluster?

1st 5er · Post by **1st 5er** » Aug 12, 2012 6:28 PM

You're welcome, and yes the top down effect is "fun".

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SOT - e30 325i fuel supply cuts out at ~40mph

SOT - e30 325i fuel supply cuts out at ~40mph