318i Riding rough and bucking when warm

[Lynx114]

Hi Everyone,

I've searched through multiple forums and all over the web for someone with the same issue but no luck so here I am.

Any help would be very very appreciated!

1988 318i M40B18 (auto but converted to manual)

In the past months the car has had new spark plugs, wires and aux belt.

The problem:

Once the car has warmed up the problems start. At low speeds and about 1/4 down on the accelerator the car starts to buck and ride quite rough. It jerks back and forth and seems like it just cuts out. It never turns off and the idle is quite stable (I cleaned the ICV completely) The problem seems to go away if I slam my foot down all the way and it feels as if it unblocks and finally takes off.

Second Problem: When i'm on the highway at about 120-140 km per hour the car just stops picking up speed and feels like there's a speed limiter or as if something suddenly get's blocked. I lift my foot wait for it to unblock it self then slam down again and off it goes but never past 140k. I now have an original exhaust but when this was happening with the sport exhaust, I would do the same thing lift off and it would sometimes backfire.

Surely a visit to the mechanic would be best but here in Munich they run about 100 Euros per hour and identifying the problem before I actually take it down to fix would be preferred.

please help lol

[Bimmerfan84]

Did you try a stomp test to see if you have any codes?


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[Lynx114]

Did you try a stomp test to see if you have any codes?


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I haven't yet. I was reading that not all EU spec cars have the possibility to do that but I will try it later today and see if anything comes up

Thanks for your reply!

[Lynx114]

I've just tried and the stomp test and it doesn't work. I had read that EU spec cars don't have this feature but please correct me if I'm wrong

[locknload]

Stomp test only works on US cars.

If you're sure you are not affected by this:http://www.m42club.com/forum/index.php?topic=7757.0, I would start by checking the easy things: throttle position switch, cam and crank position sensors, AFM, intake leaks, fuel filter and regulator and fuel pressure. Also, if the fuel supply/return lines were disconnected during the tranny swap, you might check to make sure they didn't get switched.

[Bimmerfan84]

I've just tried and the stomp test and it doesn't work. I had read that EU spec cars don't have this feature but please correct me if I'm wrong

I think your best bet is to see if you can find someone with a code reader that can read the old Bosch Motronic. Euro cars did not come with a check engine light (this can be modified to flash codes but takes some work). Sorry about that, wasn't thinking.

Does your car have a catalytic converter out of curiosity? Honestly, it sounds like a failing coil or aging ignition components or fuel supply, but it could also be a TPS sensor, Crank sensor, or an AFM that is failing.

I mean normally most people get the car scanned/diagnosed and then fix it themselves but if you don't have many tools it can be difficult.

Honestly we really need more information to help diagnose the car. Do you have the tools to do any diagnosing yourself? Please don't be offended by anything I'm saying, just trying to gauge what your skill set is so I can give you tests to perform based on what you have and what you know.


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[Lynx114]

Thanks very much for all the answers guys! @Bimmerfan84 I am certainly not offended. I have read about connecting an LED to a specific pin on the ECU and this will allow me to try the stomp test but I'd like to avoid that if I could.

I've tried testing the MAF but it seems like it's working just fine.

My next test is going to be the AFM followed by checking for vac leaks and finally I will try to have a look at all the injectors.
Experience wise, all work I've done on my cars in the past have been via help of various forums so I've been learning with time (29 now, working on my own cars for 8ish years)

I need to check where the crank and tps sensors are and that could be the following steps.

Really hope to get to the bottom of it because it's really starting to be come annoying and unpleasant to drive every day

@locknload I've read through all 9 pages of the forum you linked and it seems it's all relative to the M42 engine. I have the M40 but it may definitely be worth a shot!
I'll check it out later tonight and feedback here if this solves my problem but based on the symptoms it sounds like it's a bit different.

onwards and upwards
thanks again guys!

[nnamssorxela]

I had a very similar issue where my car ran great until it warmed up. The MAF was adding to the problem, as were some vacuum leaks, but the main problem was a faulty gas tank vent valve. When the car is running poorly, what happens when you open the gas tank? Is there suction?

On your car it is part 19 in the diagram here: Real OEM

[Lynx114]

Hi @nnamssorxela, I haven't actually tried that but I will tonight when I get home since the car will be nice and warm. I tried unplugging the maf after a drive home one day with the car off. Then turned on the car right away and it would just stall after 5 seconds of being on. From my understanding and from reading on forums this should mean that the MAF is working correctly. I've heard of people who's car was running rough and after unplugging the MAF it would stabilize but mine just turns off.

Not sure if it's helpful at all but when I bought the car it had a custom ECU chip that was running extremely rich. Apparently it would learn from your driving and adjust accordingly. After a trip from London to Munich the car was running horribly so I removed the ECU chip and replaced with original. While it was still on during the trip I pulled over for a few minutes and when i took off again between 2nd and 3rd at 4k+ rpm the car backfired and a small cloud of black came out of the exhaust. Upon arrival I checked my plugs and plug 3 was totally screwed/blackened and damaged. I obviously replaced all plugs with fresh NGK's and even replaced the lead pack but now face these problems. I've heard of E30s running rough on wrong plugs so just in case I'll be checking the new plugs which have been in for a month and if one of the 4 is obviously damaged I could be in for more serious problems.

[Elvir]

I had similar problems with my M40: Rough/bad idle, poor acceleration...

Solutions: Clean ICV, MAF, change EVAP valve, Blue sensor, Fix all vacum hoses and put Mobil 5w50. After that, engin works very fine with very smooth ride.

[Kershaw]

In this order, change and then test drive:

1. Throttle Position Sensor
2. Crankshaft Position Sensor
3. Coolant Temp Sensor (the 2 pole version.)

[Lary]

When you check your plugs ohm out the wires to make sure the plug connectors are securely connected. I had a bad connection on new wires. Also vac leaks...thorough check!