Car feels like it's intermittently braking, but its not the brakes, ideas???

[TomRat]

Ok Guys so buckle up, this is kind of lengthy, but I'll cut to the chase.

I've bought a new '87 325e, manual, LSD etc, NO ABS from factory, I bought that car with the assumption it had a sticky caliper as during driving the car would feel like it would slightly grab the brakes, then release a second later, grab, release, this was predominately felt at low speeds (0-50kph / 0-30mph.) When jacked up the front left wheel was very hard to turn, almost locked. To cut a long story short I did the following to remedy the issue:

-Bleed the brakes, no difference
-Check the lines, all good, no corrosion
-Full flush of brake system, no difference
-Rebuild the caliper completely, this slightly freed up the wheel
-New brake disc/rotor, this freed it up a little more
-New brake pads, no difference.

I did all this brake work and not once did I think "hmmm, I wonder if it's not the brakes!"

So I drove the car again, same issue, car felt like it was braking, then release, braking and releasing, not enough to kill the RPM of the motor, but enough to definitely feel the car slowing down slightly, I swear it's a brake issue because that's exactly what it feels like! The braking feeling progressively disappears the higher I go in the gears, eg: first is quite bad, second slightly better, third mostly gone. So I'm driving the car, push the clutch in, boom, issue gone! What?

So I thought ok, it's not brakes, it shouldn't be motor as it's running fine, can't be LSD as when the clutch is in the diff is still spinning and the issue is gone, so I'm thinking perhaps gearbox? So I changed gearbox oil today, and noticed a significant amount of metal shavings on the magnetic drain plug, however I'm not overly alarmed as I assume this is normal for a 30 year old car. Oil change resulted in no difference to the issue I'm having. I'm thinking possibly clutch? As the catch point is quite high and I'm not sure if it's ever been replaced.

Can someone please point me in the right direction as I'm totally lost and at my wits end, any help is super appreciated!!!!

[TomRat]

Just to reiterate the issue is NOT brake related even though it feels exactly like that. The feeling can best be described as a slowed down warped rotor, grab, release, grab, release. I have replaced or rebuilt every possible brake component that could be the cause of this issue haha. I also can't hear any grinding or anything that could point toward it being a wheel bearing or alike!

[Jaxx_]

I think you're on the right track assuming it isn't the brakes. The main clue for me is the fact that with the clutch in, it stops. That would lead me to assume that the issue lies before the input shaft of the transmission. I suspect the left wheel issue was/is unrelated.

Turn the engine over by hand with the trans in neutral and spark plugs out to see if it has any hard spots/binding. This will at least tell you the issue lies forward of the transmission. If you find binding, I would remove accessories driven by the crank. If no change, separate the transmission and engine, inspect flywheel and pressure plate, along with other clutch components. If you find nothing, there may be an issue with the rotating assembly in the engine.

[TomRat]

I think you're on the right track assuming it isn't the brakes. The main clue for me is the fact that with the clutch in, it stops. That would lead me to assume that the issue lies before the input shaft of the transmission. I suspect the left wheel issue was/is unrelated.

Turn the engine over by hand with the trans in neutral and spark plugs out to see if it has any hard spots/binding. This will at least tell you the issue lies forward of the transmission. If you find binding, I would remove accessories driven by the crank. If no change, separate the transmission and engine, inspect flywheel and pressure plate, along with other clutch components. If you find nothing, there may be an issue with the rotating assembly in the engine.

Thanks for the tips! I'm now 99% sure brake issue is unrelated. Will do what you've listed above and report back!

[TomRat]

I think you're on the right track assuming it isn't the brakes. The main clue for me is the fact that with the clutch in, it stops. That would lead me to assume that the issue lies before the input shaft of the transmission. I suspect the left wheel issue was/is unrelated.

Turn the engine over by hand with the trans in neutral and spark plugs out to see if it has any hard spots/binding. This will at least tell you the issue lies forward of the transmission. If you find binding, I would remove accessories driven by the crank. If no change, separate the transmission and engine, inspect flywheel and pressure plate, along with other clutch components. If you find nothing, there may be an issue with the rotating assembly in the engine.

So I removed all plugs before I realised there was no way I was turning the thing over by hand, there was no where I could fit my hand between the motor and the rad let alone a socket, plus my car is an mtech so it has the under body panel preventing me from coming in from below, so I decided just to crank the motor with the starter motor to see if it was struggling anywhere. Starter was healthy and had a consistent sound the whole time it was turning the motor, next ideas?

[Todd Black 88]

Does it feel like your driving into a gust of wind, and only when at a steady throttle input? Not when accelerating?

Still running the two pump fuel system? Check the in tank transfer pump is pumping.

[TomRat]

Does it feel like your driving into a gust of wind, and only when at a steady throttle input? Not when accelerating?

Still running the two pump fuel system? Check the in tank transfer pump is pumping.

Uh I'd say it feels pretty well perfect when it's not doing the "braking" issue, nothing like a gust of wind, it's most noticeable at steady throttle input, I'm not sure about the pump system, how would I know what my car has? Thanks for the reply mate!

[Todd Black 88]

Yeah, that’s what I mean. Drives fine, a heavy gust of wind slowing you down, lets up, drives fine, gust of wind......repeat.

[flumph]

Shot in the dark, but maybe there's a belt or some accessory intermittently binding up? If it was the alternator or AC compressor and a bearing or a pulley was binding up (maybe a pulley is on loose and it's flopping around?) then you would only notice it while each of those devices was functioning under load. Would explain why you don't hear any rough spots when you crank it with no plugs.

Like maybe your battery cable insulation wore through in a spot, and when it vibrates just the right way, you get a current path from the alternator back to ground through the body, the alternator gets a lot harder to turn, and it loads down the engine. This also wouldn't blow a fuse because there isn't a fuse on the main battery cable.

Or, or, maybe, there's a bad gasket or a leak somewhere that intermittently spooges coolant into your intake plenum. Or maybe you have bubbles in your coolant system that run by the temp sensor and give the DME a nonsensical reading. Are you losing coolant?


Is there metal in your oil?

Just some ideas.

[TomRat]

Yeah, that’s what I mean. Drives fine, a heavy gust of wind slowing you down, lets up, drives fine, gust of wind......repeat.

Oh yeaahhh, I guess that's how it could feel, but a very rapid gust of wind, we're talking 0.5 seconds you know, but that would be a semi good way to describe it!

[D.Martijn]

I think I can relate to your problem. I've also had the feeling that the car was braking and then accelerate again, brake... and so on. Like Todd describes it, is spot on. It feels like a gust of wind is hitting you.

I might sound stupid saying this but when I encountered this I had my AC turned on although it was freezing outside; I had it mistaken for the air recirculating air button...
I haven't driven with my AC on since I noticed that I pushed the wrong button and I haven't had this problem/feeling anymore of the car braking.

[TomRat]

I think I can relate to your problem. I've also had the feeling that the car was braking and then accelerate again, brake... and so on. Like Todd describes it, is spot on. It feels like a gust of wind is hitting you.

I might sound stupid saying this but when I encountered this I had my AC turned on although it was freezing outside; I had it mistaken for the air recirculating air button...
I haven't driven with my AC on since I noticed that I pushed the wrong button and I haven't had this problem/feeling anymore of the car braking.

Thanks for the tip man, but my A/C is out of gas so I don't drive with it on ever, I guess that's not my problem :/

[TomRat]

For anyone still following I did some more testing today, the issue seems to only occur when the motor is driving the wheels, but not when the wheels are driving the motor, for example if the car is in 4th gear doing 20kph just rolling, the wheels are driving the motor, the issue is not present, but as soon as I drop it back a gear or two and give it some gas to get the motor to drive the wheels the issue comes back. Once again with the clutch pushed in just coasting, or the care in neutral rolling, the issue is gone. I have noticed some "clunking" when shifting gears coming from the back end, could this be related? TIA!

[TomRat]

The RPM's also do not dip when the issue happens.

[88BlackS-ETA]

I have the same problem with my auto 2.7L. I can hear it, feel it but RPM stays the same. Only happens between 1500-3000 RPM. Its been doing it for over a year, I just didnt drive it much but now I have a job again and its my only car so I'll be following this. I do have a squeak somewhere upfront now on the engine.