1992 BMW 730i Auto accelerates when engine is idling
Discussion
I have a lovely old BMW 1992 730i which I've owned for about 15 months now. I enjoy wafting about in it, as my other cars are all manuals and have a fairly stiff ride, while this thing with its 15" wheels is like driving a four poster bed. Before buying it last year, the last automatic I had was a 1984 W123 Mercedes 230E, which while a decent drive, had a smaller four cylinder engine and god knows what transmission. I have experience driving other automatics including modern cars with the ZF8, and also Porches with PDK (if you count that as an automatic).
Anyway let me get to the point. One thing I've noticed with the 730i which I haven't seen with any other automatic car is that you have to use a decent amount of brake pressure to hold it stationary in Drive. It's not that the brakes are weak - they work well. But the car just wants to go. In other automatics I've driven and certainly in the Merc, you only needed light pressure on the brake to hold it stationary. And when you do release the brake, it doesn't just roll at 3-4 mph like other automatics. It'll accelerate to between 10 and 15 mph, and if there is the slightest hint of a downhill slope, it'll change to second gear at 15mph and then accelerate to about 20-25 miles per hour. And if you're driving at 30mph on a flat surface, it'll just carry on doing that if you take your foot off the accelerator. Basically there is no engine braking below 30mph.
Another thing I've noticed is that the engine idles at around 800 rpm in neutral, but if I put it in drive, it seems to settle at around 1100-1200rpm once the car has reached whatever equilibrium speed the slope allows. I've tried other gears and none make a difference. ATF is at the correct level and seems okay, by which I mean it's the right transparency, viscosity and colour and it doesn't smell burnt.
What controls automatic gearbox torque? Is it just engine rpm, or does ATF type have a bearing on this? Why does the engine idle at a higher speed when the transmission is in Drive? The transmission could probably do with a bit of a service as it gives a very small shunt when you put it in drive and gear changes aren't fully smooth until everything warms up. But I can't see how that would affect the torque at idle engine speeds. Do I need to adjust the engine idle down somehow?
The engine is the M30B30 185bhp 3.0 straight six, and the transmission is the ZF 4HP22 Type A.
Thanks for your thoughts.
Anyway let me get to the point. One thing I've noticed with the 730i which I haven't seen with any other automatic car is that you have to use a decent amount of brake pressure to hold it stationary in Drive. It's not that the brakes are weak - they work well. But the car just wants to go. In other automatics I've driven and certainly in the Merc, you only needed light pressure on the brake to hold it stationary. And when you do release the brake, it doesn't just roll at 3-4 mph like other automatics. It'll accelerate to between 10 and 15 mph, and if there is the slightest hint of a downhill slope, it'll change to second gear at 15mph and then accelerate to about 20-25 miles per hour. And if you're driving at 30mph on a flat surface, it'll just carry on doing that if you take your foot off the accelerator. Basically there is no engine braking below 30mph.
Another thing I've noticed is that the engine idles at around 800 rpm in neutral, but if I put it in drive, it seems to settle at around 1100-1200rpm once the car has reached whatever equilibrium speed the slope allows. I've tried other gears and none make a difference. ATF is at the correct level and seems okay, by which I mean it's the right transparency, viscosity and colour and it doesn't smell burnt.
What controls automatic gearbox torque? Is it just engine rpm, or does ATF type have a bearing on this? Why does the engine idle at a higher speed when the transmission is in Drive? The transmission could probably do with a bit of a service as it gives a very small shunt when you put it in drive and gear changes aren't fully smooth until everything warms up. But I can't see how that would affect the torque at idle engine speeds. Do I need to adjust the engine idle down somehow?
The engine is the M30B30 185bhp 3.0 straight six, and the transmission is the ZF 4HP22 Type A.
Thanks for your thoughts.
While you say it seems OK, a transmission service may be a good start point.
My (limited) understanding of autos is that it is the torque converter that makes the car move by hydraulically pumping fluid from input to output turbine.
Put simply one turbine is spinning with the engine & the other is moved by the fluid it transfers. Therefore, the resultant torque/speed of the output is set by manufacture.
IIRC a "high stall" (name could be wrong) Torque converter is used in a more sporty car to accelerate at higher revs for a quicker get-away so perhaps a "low stall" Torque converter is used in a car designed to be more wafty, not sure.
However, if there is too much/little fluid it may affect the process.
Also, you say that the idle lifts in D, others with similar cars on here will know if this is right, I don't, but perhaps a transmission/BM specialist might also know more.
My (limited) understanding of autos is that it is the torque converter that makes the car move by hydraulically pumping fluid from input to output turbine.
Put simply one turbine is spinning with the engine & the other is moved by the fluid it transfers. Therefore, the resultant torque/speed of the output is set by manufacture.
IIRC a "high stall" (name could be wrong) Torque converter is used in a more sporty car to accelerate at higher revs for a quicker get-away so perhaps a "low stall" Torque converter is used in a car designed to be more wafty, not sure.
However, if there is too much/little fluid it may affect the process.
Also, you say that the idle lifts in D, others with similar cars on here will know if this is right, I don't, but perhaps a transmission/BM specialist might also know more.
Edited by E-bmw on Friday 14th October 09:21
What's the idle rpm when in gear with the brakes on?
Sounds like your idle speed is too high.
The Classic Range Rovers used the ZF4HP22 box & there can be issues with the engine idle control valve (aka stepper motor)
This might be of interest: www.landrovermonthly.co.uk/articles/classics/diagn...
I wonder if yours has something similar?
Perhaps a diagnostic?
Sounds like your idle speed is too high.
The Classic Range Rovers used the ZF4HP22 box & there can be issues with the engine idle control valve (aka stepper motor)
This might be of interest: www.landrovermonthly.co.uk/articles/classics/diagn...
I wonder if yours has something similar?
Perhaps a diagnostic?
Edited by paintman on Friday 14th October 12:08
Your engine is 'idling' at 1,200 rpm. In a long geared 4 speed auto, that's easily going to be 25mph in 3rd etc. as you're experiencing.
Personally I don't think it's your gearbox or the torque converter. Your idle is much too high, so from the torque converter / gearbox's perspective, you're cruising at 1,200rpm...
In every auto I've experienced, in neutral (thus minimal load on the engine) the idle is higher than in drive on the brakes.
The above post is right, think of a torque converter as two fans in hydraulic fluid. One is spun by the engine, the other connected to the input shaft of the gearbox.
The engine spinning the 'fan' spins the hydraulic fluid, which then spins the 'fan' on the gearbox. If the gearbox is in neutral, that 'fan' can just spin freely. Put it in D and the gearbox is in gear, if you're holding it on the brakes, then the gearbox's 'fan' inside the torque converter is held stationary and thus resists the spinning hydraulic fluid, and puts resistance on the engine, dragging the idle down (normally!)
Personally I don't think it's your gearbox or the torque converter. Your idle is much too high, so from the torque converter / gearbox's perspective, you're cruising at 1,200rpm...
In every auto I've experienced, in neutral (thus minimal load on the engine) the idle is higher than in drive on the brakes.
The above post is right, think of a torque converter as two fans in hydraulic fluid. One is spun by the engine, the other connected to the input shaft of the gearbox.
The engine spinning the 'fan' spins the hydraulic fluid, which then spins the 'fan' on the gearbox. If the gearbox is in neutral, that 'fan' can just spin freely. Put it in D and the gearbox is in gear, if you're holding it on the brakes, then the gearbox's 'fan' inside the torque converter is held stationary and thus resists the spinning hydraulic fluid, and puts resistance on the engine, dragging the idle down (normally!)
I guess the engine management system is designed to open the throttle slightly to stop the engine revs dropping too low while it is in gear while stationary.
The symptoms suggest that the mechanism is too effective. If you can figure out what the mechanism is, you might be able to work out why that is. I suppose the most likely mechanism is an additional air valve. Perhaps that's working in the wrong range of positions, or missing a restrictor, or introducing an air leak, or something like that.
The systems I'm familiar with often use external hoses around the outside of the throttle body for the additional air supply. With these you can squash each hose flat to see whether it's passing air when you see the problem. If/when you have confirmed that it's an excess air flow problem and where the air flow is coming from then it should be simple enough to restrict that air flow.
The symptoms suggest that the mechanism is too effective. If you can figure out what the mechanism is, you might be able to work out why that is. I suppose the most likely mechanism is an additional air valve. Perhaps that's working in the wrong range of positions, or missing a restrictor, or introducing an air leak, or something like that.
The systems I'm familiar with often use external hoses around the outside of the throttle body for the additional air supply. With these you can squash each hose flat to see whether it's passing air when you see the problem. If/when you have confirmed that it's an excess air flow problem and where the air flow is coming from then it should be simple enough to restrict that air flow.
Thank you guys - you've reminded me of something. I remember that within a week of buying the car unplugging the idle air control valve because the engine was surging after a cold start for 20-30 seconds. Unplugging it made it run smoothly. But after plugging it in when warm, it made zero difference to the idle speed or quality, so I think I left it unplugged. I'd completely forgotten about that but perhaps it's the source of the issue and if left plugged in, the ECU will recalibrate after some time. Let me check that and get back to you. I don't know if the valve itself is faulty but I've heard that DIY cleaning *can* work wonders, but if it's faulty I'm sure a replacement should be easy enough to find.
Either buy a Haynes manual fore your car, or this
https://www.amazon.co.uk/Automotive-Engine-Managem...
For a 92-94 &30i, your idle speed should be 600rp,m +/-50, CO 0.7 +/- 0.5
It also gives various details you could work through. I'd assume the model specific Haynes manual does the same, but the above linked book covers a lot of old management systems
Most likely as has been indicated someone has likely f
ked about with something botching things over the years, instead of ensuring it all runs correctly. Could be a faulty ISCV, could be throttle maladjusted, could be an air leak, could be too rich, could be many things.
https://www.amazon.co.uk/Automotive-Engine-Managem...
For a 92-94 &30i, your idle speed should be 600rp,m +/-50, CO 0.7 +/- 0.5
It also gives various details you could work through. I'd assume the model specific Haynes manual does the same, but the above linked book covers a lot of old management systems
Most likely as has been indicated someone has likely f
ked about with something botching things over the years, instead of ensuring it all runs correctly. Could be a faulty ISCV, could be throttle maladjusted, could be an air leak, could be too rich, could be many things.Edited by stevieturbo on Friday 14th October 19:34
Polly Grigora said:
Horrible job to get it sorted if it's nothing obvious. Has DME plus switchable transmission/ECU, potentiometer at the pedal, fly by wire hell at its best
Wouldn't wish to take a guess, even with all information at hand findings don't always make any sense depending on how new or old the information is, having access to very accurate information is critical especially for setting up the throttle potentiometer, after all is said and done there's still lots of ECU fun to be had
Wishing you an easy fix
fly by wire ? It's a basic 1992 BMW.Wouldn't wish to take a guess, even with all information at hand findings don't always make any sense depending on how new or old the information is, having access to very accurate information is critical especially for setting up the throttle potentiometer, after all is said and done there's still lots of ECU fun to be had
Wishing you an easy fix
stevieturbo said:
Polly Grigora said:
Horrible job to get it sorted if it's nothing obvious. Has DME plus switchable transmission/ECU, potentiometer at the pedal, fly by wire hell at its best
Wouldn't wish to take a guess, even with all information at hand findings don't always make any sense depending on how new or old the information is, having access to very accurate information is critical especially for setting up the throttle potentiometer, after all is said and done there's still lots of ECU fun to be had
Wishing you an easy fix
fly by wire ? It's a basic 1992 BMW.Wouldn't wish to take a guess, even with all information at hand findings don't always make any sense depending on how new or old the information is, having access to very accurate information is critical especially for setting up the throttle potentiometer, after all is said and done there's still lots of ECU fun to be had
Wishing you an easy fix
Feeling like a proper idiot now
I had an 88 735 and idle was around 700 to 800 in neutral and didnt really rise in gear. Could be a bad iacv, tps, an ecu or wiring fault or an air leak. There is a lot of scope for perished hoses or seals on a complicated car that age.
I cant think of anything in the gearbox that would cause the idle to lift like that.
Could be a split in the hose from afm to throttle body, a split iacv hose, carbon on the throlle plate.
You said you disconnected the iacv, that could be masking other issues. Depending what version of motronic it is the iacv works in different ways, some have a pwm signal some have a stepper motor, i think yours is too late for a thermotime valve. 92 would be around when they got more advanced managment for the cats.
Jealous of the old bus, regret parting with my old 7s
Looking back the e23 was the best handling by a mile. I had a couple of e32s acouple of e38s and a e65. The e65 was the only disappointing one.
I cant think of anything in the gearbox that would cause the idle to lift like that.
Could be a split in the hose from afm to throttle body, a split iacv hose, carbon on the throlle plate.
You said you disconnected the iacv, that could be masking other issues. Depending what version of motronic it is the iacv works in different ways, some have a pwm signal some have a stepper motor, i think yours is too late for a thermotime valve. 92 would be around when they got more advanced managment for the cats.
Jealous of the old bus, regret parting with my old 7s
Looking back the e23 was the best handling by a mile. I had a couple of e32s acouple of e38s and a e65. The e65 was the only disappointing one.
Edited by OldGermanHeaps on Friday 14th October 23:10
Okay so I did a bit more investigation last night. The IACV was indeed plugged in, but disconnecting it just raises the idle speed when the engine isn;t warmed up. Once warmed up, it doesn't seem to make any difference to the idle speed at all. Once thing I've notices is that the IACV makes a buzzing noise when plugged in - sort of like a fluorescent tube which is on the way out. I don't know if that means that its motor is stalled or if it means it's trying to push against a stop. Or perhaps it means nothing and they all do that. I checked all the rubber hoses and they seem fairly supple and any cracks or leaks are not obvious. I don't have a smoke tester. I gather you can make one fairly easily, but I'd prefer to buy one inexpensively. Any recommendations? I do have an air compressor. Actually I have two!
Sounds like you need to check base idle settings first checking butterfly air gap inc idle bypass bleed screw setting , also remember that if the butterfly stop screw is tampered with causing butterfly to be cracked open to raise idle speed previously for example then the ECU/DME may not being getting its correct idle ref position from the throttle position switch/potentiometer etc etc , go through it all methodically it may be old but was still advanced for its time inc potential to screw up settings , Bosch Motronic IIRC 
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