1985 535i high & pulsing idle

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Blades74
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Joined: Aug 12, 2020 7:25 PM
Location: augusta, georgia

1985 535i high & pulsing idle

Post by Blades74 »

Good Day e28 fans! New to the site, and have enjoyed exploring the vast amount of know-how available. I have what appears to be a common idle issue. So far, prior posts have not been helpful. Any help would be greatly appreciated.
Have recently purchased a 1985 535i manual. Original problem was low idle, stalling and rich mixture. I replaced the AFM. This was a big help, but now idle is high throughout the temp range. At operating temp, I get the pulsing idle problem between about 900 and 1200RPM. The following have been replaced, prior to the development of this particular issue:
Fuels pumps, fuel regulator, injectors, distributor/rotor, wires, plugs, valve cover, vacuum lines, ICV, temp sensor, temp switch, cold start valve, AFM, throttle boots, oxygen sensor.
So as you can see, there isn’t too much left to replace. Would love to fix this without replacing the Motronic. Thanks guys!
RetiredDoc
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Re: 1985 535i high & pulsing idle

Post by RetiredDoc »

Idle control module - but first check the continuity of the wires running from it to the idle control valve.

Programma rebuilt ICM is just a bit over $100.
cek
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Re: 1985 535i high & pulsing idle

Post by cek »

Blades74 wrote: Oct 21, 2020 9:34 AM Good Day e28 fans! New to the site, and have enjoyed exploring the vast amount of know-how available. I have what appears to be a common idle issue. So far, prior posts have not been helpful. Any help would be greatly appreciated.
Have recently purchased a 1985 535i manual. Original problem was low idle, stalling and rich mixture. I replaced the AFM. This was a big help, but now idle is high throughout the temp range. At operating temp, I get the pulsing idle problem between about 900 and 1200RPM. The following have been replaced, prior to the development of this particular issue:
Fuels pumps, fuel regulator, injectors, distributor/rotor, wires, plugs, valve cover, vacuum lines, ICV, temp sensor, temp switch, cold start valve, AFM, throttle boots, oxygen sensor.
So as you can see, there isn’t too much left to replace. Would love to fix this without replacing the Motronic. Thanks guys!
FWIW, I had a similar gremlin way back and did everything you did including replacing the ECU. That didn't fix.

Have you adjusted your valves?

Is the vacuum for the interior temperature sensor plugged in or capped?
Blades74
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Location: augusta, georgia

Re: 1985 535i high & pulsing idle

Post by Blades74 »

Thanks guys. So i have done a valve adjustment. They were almost all very close. Like it had been done recently. I admit, I haven't done many of them. So hopefully I didn't screw it up. But i don't think that's it, because is has good power at speed. Very drive-able. Valves do make a good bit of noise.

I did try the interior temp sensor. Hard to tell there, it's such a small amount of vacuum. I might need to look again.

This one has it's ICM integrated into the Motronic, not s seperate unit. I'll try to check continuity there.

So i happen to have an extra ICV. I have hooked it up with engine is cold, in place of the one installed. So i can see inside the ICV valve. I turn the ignition when cold and take a look. It's at 3/4 open. I've done this with engine warm, and it's about 1/2 open. And it stays there.

I almost wonder if the engine control is giving it too much fuel when cold. And later, which causes pulsing as it burns off. Anyone heard of that?
tn535i
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Re: 1985 535i high & pulsing idle

Post by tn535i »

It's hard for me to believe enough hasn't been written about idle control. Have you really checked all of it out and can you tell us what you've done and the results of some of the simpler tests?

You are correct there is no separate ICM. Willing to help but seems like you could have something as simple as TPS you have not tested. And when you replace the AFM you really need to go through the bypass adjustment process.
Blades74
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Location: augusta, georgia

Re: 1985 535i high & pulsing idle

Post by Blades74 »

The interior temp gauge is very worn. So I'm replacing. However the vacuum pull on the line was strong, and it didn't effect running when i plugged it.

I tested the TPS a few times. The signal has always checked out properly. I have visually inspected the throttle plate. It seams okay. Speaking of throttle, another symptom coincides with the intermittent idle pulse. I have manual trans. When the problem occurs, I have significant jerking/bucking when getting on and off the throttle. Almost like the engine is completely starved of idle air at these times.

I test drove the car two days ago after replacing master and slave clutch cylinders. Idle was relatively low and smooth, and the transmission never shifted more smoothly. I was happy! Yesterday, all the problems were back!
Blades74
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Re: 1985 535i high & pulsing idle

Post by Blades74 »

I have re-tested all the relevant sensor connections at the ECU. All connections are good and resistance in the proper ranges.

I suspected the ECU wasn’t sending proper signals to the ICV (3-wire). So I tested the connector/plug and confirmed 12V at the center terminal as suggested by Bentley. But that’s where Bentley stops. Haynes goes further suggesting there should be 10V at both of the outer 2 terminal connections. I tested these and one has voltage, the other does not. Is this possible without a faulty ECU?

The wiring schematic shows one outer connection controls opening the valve, and the other closes. Perhaps the valve isn't restricting once the engine is up to temp?
tn535i
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Re: 1985 535i high & pulsing idle

Post by tn535i »

Haynes is wrong... you won't ever have 10 volts on each of the outer connections. IME if you test the two outer terminals (each to ground independently) it might add up to about 10v with maybe one at 6 and the other 4. If you have no voltage on one side it could be bad ICV or driver in the DME.

The DME grounds the outer terminals (one retracts and the other extends) to balance the position of the ICV which is why it vibrates and hums as the two directions are fighting each other (think push pull) but it floats near a midpoint. If it does not do that it's not working.

You can test the resistance from center to each outside terminal to see if either coil is bad. They should be nearly identical. You can also apply 12v with jumper to center and ground the opposite sides to see that it flips from open to close to test it. If all that checks out it's likely the DME or less likely the wires.
Blades74
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Re: 1985 535i high & pulsing idle

Post by Blades74 »

That makes more sense than both outer terminals having the same voltage. All the resistance tests passed, both at the ICV and at the ECU connection.

Since I have a spare ICV, I decided to drive to the car until the idle pulsing was strong, and then plug the spare into the terminal and watch the valve while running. To my surprise, the spare valve oscillated 1/2 to 3/4 closed in conjunction with the actual idle pulsing. Meanwhile the unplugged ICV was in the throttle body doing nothing. In other words, when plugged in, the ICV unsuccessfully oscillates open/closed to try to halt the pulse. So the pulse must be unrelated to the ICV?

The prior owner installed new fuel pumps, FPR and injectors less than a year ago. I’ve assumed those were in working order. Should I run a fuel pressure test next?

My exhaust probably has a leak. Would that cause the idle issues?
tn535i
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Re: 1985 535i high & pulsing idle

Post by tn535i »

We hear about these weird pulsation issues and people struggling to resolve them and I have two experiences. I've seen it when an AFM is worn on one spot (usually were it idles) causing it to hunt back and forth across that bad spot and I've seen it when an O2 sensor is bad or slow responding driving mixture to an extreme where the speed falls but idle control tries to bring it back up and then cycle repeats.

Your best diagnostic may be running open loop but watch the response from a known good O2 sensor. But first just unplug the O2 sensor to force it open loop and if the problem goes away it's O2 and idle base mixture related. Might need a new O2 and then adjust of the idle air bypass to get it just rich and should fix it.

I would also test the AFM which you can read about and you should test response with voltage (NOT resistance) using a 9v or the car battery as power source... it must be really smooth and if you find a flat spot or spike in voltage were it idles then try re-tracking the AFM. It's critical and most critical if you let the car run open loop.

Bad DME is another possibility but more remote. Perhaps even just poor connection/solder joints.
Blades74
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Re: 1985 535i high & pulsing idle

Post by Blades74 »

The AMF, and the O2 were both changed a couple months back. They needed it. The idle pulsing issue didn't exist at the time they were giving problems. They caused a very rich mixture. It ran well for a time after they (and most everything else) were changed.

I might have to give up and take it into the shop. Admit defeat. Only other option I see is confirming the new fuel delivery system is working properly. Or I send the ECU into ProgRama for a rebuild.

Since it ran so rich for a while, I wonder if the Cat could have gone bad. Would that effect idle?
cek
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Re: 1985 535i high & pulsing idle

Post by cek »

Blades74 wrote: Nov 05, 2020 10:39 AM The AMF, and the O2 were both changed a couple months back. They needed it. The idle pulsing issue didn't exist at the time they were giving problems. They caused a very rich mixture. It ran well for a time after they (and most everything else) were changed.

I might have to give up and take it into the shop. Admit defeat. Only other option I see is confirming the new fuel delivery system is working properly. Or I send the ECU into ProgRama for a rebuild.

Since it ran so rich for a while, I wonder if the Cat could have gone bad. Would that effect idle?
If the cat is clogged....

Do not admit defeat yet! You have this! Even with COVID, I bet there are members in Atlanta who would love to do a wrench session to help you diagnose this.
Blades74
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Re: 1985 535i high & pulsing idle

Post by Blades74 »

There are a few additional symptoms that occasionally occur. On 3 or 4 occasions, I've been heavy on the accelerator, goosing it up a hill, and it has a brief total power loss. I let off the gas, and everything is back to normal.

And I would describe the power going up the rev scale as less than even. Sort of like some very minor spots of hesitation, typically between 2k and 3.5k RPM

My economy gauge is erratic. When the engine is cold and running okay, it's pegged at 0, and stays there unless I'm totally coasting. When the surge is happening, the gauge will swing back and forth from 40 to 0, even while at a total stop. I've read the gauge operates off fuel flow info from the ECU.

The pulsing issue by no means happens 100% of the time. Every so often, I'll take it out for a 30 minute drive, and nothing goes wrong.

All my sensors are either new, or have been confirmed multiple times. Fuel delivery is all new, but I haven't tested it. I have two ICVs, and they both work.
Blades74
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Re: 1985 535i high & pulsing idle

Post by Blades74 »

I sent the ECU off to ProgRama yesterday. We'll see.
tn535i
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Re: 1985 535i high & pulsing idle

Post by tn535i »

Blades74 wrote: Nov 06, 2020 7:37 AM There are a few additional symptoms that occasionally occur. On 3 or 4 occasions, I've been heavy on the accelerator, goosing it up a hill, and it has a brief total power loss. I let off the gas, and everything is back to normal.

And I would describe the power going up the rev scale as less than even. Sort of like some very minor spots of hesitation, typically between 2k and 3.5k RPM

My economy gauge is erratic. When the engine is cold and running okay, it's pegged at 0, and stays there unless I'm totally coasting. When the surge is happening, the gauge will swing back and forth from 40 to 0, even while at a total stop. I've read the gauge operates off fuel flow info from the ECU.

The pulsing issue by no means happens 100% of the time. Every so often, I'll take it out for a 30 minute drive, and nothing goes wrong.

All my sensors are either new, or have been confirmed multiple times. Fuel delivery is all new, but I haven't tested it. I have two ICVs, and they both work.
That first thing sounds like a loss of signal or power somewhere like the injector drivers (DME) which can cause a complete loss of power even if it keeps running but very badly. Could very well be the DME.

Some perceived slight hesitation or change as you pass through the range where it transitions from closed to open loop is almost normal on these cars. Some describe it as coming onto cam at a higher rev range than they normally use. The only way I know to tune this a little better is to watch the O2 sensor and see the response and use the AFM spring to bring it back closer to stoic. Too rich in the range then tighten the spring a click and watch it again. Too lean loosen the spring. You have a new AFM but it does NOT mean it's perfect tune for this motor.

The idle air bypass is again a function of the particular motor and not necessarily correct with a new AFM. It should be set so it is just slightly rich with the O2 unplugged at idle and easy to do but before you do any of that check again and double check for ANY vacuum leaks.

The econometer must be electrical in nature and so again maybe sending the DME off is a good move. But you describe a swing from either 0 to full scale and never in between. Almost makes me wonder if more than DME alone, perhaps the wrong coding plug in the tach/econometer cluster?
Blades74
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Location: augusta, georgia

Re: 1985 535i high & pulsing idle

Post by Blades74 »

The ECU came in yesterday. Idle is now lower, and so far no pulsing. Very pleased; knock on wood.

I hit the gas. Great acceleration, but it lost power at around 3,500. Came back as soon as I released then pressed the gas pedal. Any ideas out there? Loss of fuel pressure? Nearly all fuel delivery has been replaced by the PO, but I haven't checked anything. Wonder if some debris may be in the lines, etc. The car was parked for a few years. Never done a fuel pressure test. Any hints?
Blades74
Posts: 14
Joined: Aug 12, 2020 7:25 PM
Location: augusta, georgia

Re: 1985 535i high & pulsing idle

Post by Blades74 »

Well damn! After 4 good joy rides, the pulsing idle came back yesterday. Very intermittent. ECU has been refurbished. All sensors are new. I read a post about loss of power due to a bad CAT causing high back-pressure. Heard of this? I do have an exhaust rattle.

And i supposed I'll need to finally confirm fuel pressure. I'm thinking a shop would be better at this than I.
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