e36 m52 Revving on its own

[casem]

I've searched everywhere for this problem and cannot find a solution.

This issue started a month or two ago, and the car has become undriveable. Randomly while in neutral, it will rev up and down from 1.5k to 3k. It will also do this in gear and it feels like the cruise control is on. If I stop the car while its doing this, it is very hard to start up again.

Initially it would only do this like once every 5th time I drove the car. Now it does it much more often, and has become worse. Sometimes when I start the car, it will rev up to 4k and hold it there. This just happened about 30 seconds after I started driving the car.

I thought it was the throttle position sensor, which I replaced and nothing changed.

Here's a video of it happening briefly. The last time I drove it, it did it for about 15 minutes while driving.

https://www.youtube.com/shorts/KdCH5k7doRo

[casem]

Also worth noting: When it was happening while driving, I was cruising in 3rd gear about 2.5k rpm. If I lift my foot, it will keep going the same speed like cruise control is on. I could give it a little gas, but if I went half throttle, even at 2.5k, the car would be violently bucking.

[Floxer]

It can only be related to things that can increase air flow into the engine:

1) Idle control valve
2) throttle: gas pedal cable or cruise control cable


Do you have cruise control?

auto or manual?

use romraider datalogger to see if the idle control valve is activated by the DME and causing the problems. Buy cable from amazon or aliexpress and download romraider for free which includes romraider datalogger. Instructions to use it are on the romraider forum.

[Eric93se]

You might have a vacuum leak, check all hoses and the intake bellows for deep cracks. You should probably smoke test the engine because the leak can be anywhere.

Not sure but I think all adaptations should be reset after changing the tps, but you should fix the problem before doing that. You can use inpa or ms41 quickflash.

[XnWarden]

My car did this exact same thing with a vacuum leak. Smoke test the intake and vacuum system

Sent from my Pixel 7 Pro using Tapatalk

[casem]

Update: I have cruise control and its a manual. I took the car into a shop and they tested to see if there were any leaks, which he said there was not. Based on the testing they did, they told me my best bet was to replace the icv. I've replaced it and it still did not solve the problem.

[E36 Em Tree]

How's your MAF? Are your throttle cables moving at all? Im curious about this thread now.

[Eric93se]

You should put inpa (free, z3 diagnostics thread) on a laptop, order a USB inpa cable and 20 pin round adapter, so you can get BMW codes read and do better diagnostics.

While the engine is running try unplugging the mafs.

[casem]

I don't know about the MAF. I'm pretty sure they read all the diagnostics at the shop and couldn't find anything.

I drove it today for about 20 minutes today and might be able to describe the problem better now.

The original video I posted is how it acts in neutral, but in gear it acts a little differently. If I'm driving down the road, I know it starts acting up because it feels like the cruise control has been turned on. It will hold the exact amount of throttle input, so I can take my foot completely off the gas.

While its doing this, if I give it more than 50% throttle and then let off completely, it engine breaks like it normally should. But, the moment I give it any gas at all, it will go back into this cruise control type thing. Sometimes it lasts only for seconds and stops, other times like today it was happening the entire drive.

[E36 Em Tree]

Do what Eric said.

But also a bad/different maf wouldn't necessarily throw a code. But a bad maf could possibly be causing your revs. There's used ones on ebay if it comes down to that though.

But also I remember when I had 2 DMEs (oem and one stage 3 dinan; maf, tb, intake) I switched the DME and MAF up and my car was kind of acting like that. Only reason why I suggested the MAF. I forgot to switch my OEM one to the one tunrd for a 3.5" MAF after smog, or vice versa, when I changed it to the oem maf and left the stage 3 tuned one (for a Porsche or 3.5" maf).

Have you checked your engine bay when the car is in neutral and revving? Is anything moving?

[Eric93se]

Remove your fuel rail and energize the fuel pump to check if they are leaking down. Put a small plastic cup under each injector.

- - - Updated - - -

Did you try unplugging the maf?

What brand is your maf?

[USCooper]

Having the same issue myself with my 97 328i track car. Cleaned the MAF, the ICV (didn't look bad and moved freely), and proactively replaced the hose from the intake elbow boot to the ICV but it didn't look that bad. Need to finish putting it back together and see if it is solved. What are the common vacuum lines that would leak? It just seemed like it was getting a ton of air to rev on its own, so I kind of assumed a leak but haven't found one.

[casem]

Update: I asked the first shop to check for any leaks, which they said they did. They said there was no leaks. They basically told me they couldn't communicate with the DME and they wanted to try and dig around to figure out why and possibly replace it. They also told me if I'm doing things myself ICV replacement is the best shot I have to fix it.

I took it in somewhere else, they smoke tested it and said there's leaks in 3 different spots. Repairs aren't done yet, but probably safe to say that's the issue.

[casem]

Another Update: Vacuum leak fixed and didn't solve the issue.

[USCooper]

Just fixed mine. I replaced the throttle position sensor first, but then went down the route you did thinking it was a vacuum leak. I cleaned the MAF, ICV (which was fine), replaced the hose from the ICV to the intake elbow, and even replaced the air temp sensor I have in the elbow (car is a non-ASC with M50 manifold conversion). Fired it up and it still idled and ran like total crap (although it didn't spike revs on me). I connected my OBD2 bluetooth adapter and was looking at data within the Torque app and noticed that even though my temp gauge on the cluster was going up while the car idled like normal, the car thought the water temperature was -40. Car had a soft code for P0116, too. So I ordered a water temp sensor and swapped that out (easy enough job at least with the M50 manifold-- just needed a 23mm deep socket and torqued the new one to spec which looked to be 13ft/lbs). Car fired right up today and was idling really well so I bled the coolant a bit (swapping the sensor can put some air in there) and monitored temps. Everything was great. Took it for a pretty long drive, went to some shops, etc., and every time it fired up and ran great.