High RPM surging/hesitation

[Brayden2544]

Working on sorting my 1987 535IS, I’ve been chasing a high rpm hesitation since I bought it. The power cuts by about 30% between 4500 and 5000 rpm until redline. In 1st gear it will pull hard to redline at times, sometimes it bogs down down. In higher gears it almost always bogs down at high rpm.

I’ve thought it’s a fuel delivery issue, but I’ve replaced everything I can think of.
New in line pump (oem Bosch)
New fuel filter (oem Bosch)
PO replaced in tank pump in a attempt to fix the issue about a year ago
Pulled all fuel injectors and changed o-rings, filters and pintles which seems to have made it even more powerful in lower Rev ranges.

Other maintenance performed that could be related
All new Bosch oem silver plugs
New Bosch dizzy cap
New Bosch dizzy rotor
New Meyle air filter
Oil change with Móbil 1
New CTS with no change, I even bypassed the plug with no change.
Valves adjusted which quieted the motor but no change

I’ve tested the TPS, unplugged it, adjusted. No change to high rpm performance.
I did a vapor test for vacuum leaks and the only place smoke came out of was the in cabin temp sensor. I’ve trief clogging it but no major difference. Unplugging the dipstick while running caused a definite change in idle speed so I don’t think it’s a vacuum leak.

Afm and CPS check out with voltmeter.

I haven’t messed with o2 sensor at all, but my understanding is that its signal is ignored at WOT.

I haven’t tested FPR with fuel pressure gauge as I dont have one, do the FPRs fail often?

Coil checks out with voltmeter.

My best guess now is possibly a loose or soon to fail CPS that goes wonky with high vibrations at high rpm?

Any help you guys have would be much appreciated.

Thanks!

[Tiit]

You should test fuel pressure during hesitation. If the fuel pressure is spot on 3 bars while the hesitation is happening at wot, it rules out fuel delivery issue. If pressure starts dropping under heavy load, its likely a weak fuel pump or restriction in the lines etc.


Probably not safe, but this is how I tested mine successfully.

IMG_9686 by Tiit Saul, on Flickr

[tn535i]

Working on sorting my 1987 535IS,

Pulled all fuel injectors and changed o-rings, filters and pintles which seems to have made it even more powerful in lower Rev ranges.

Thanks!

Mine was doing similar a long time back. There is more load in any higher gear that 1 both on the ignition and fuel delivery. I've had weak ignition cause this but had replaced all that and verified fuel delivery. So I'll suggest replacing o-rings and filters is not the same as having the injectors cleaned and tested. I think it only takes one or two to cause problems also. Mine were also leaking/sweating at the body when cold so I eventually replaced mine with the Bosch Ford V8 injector and all the problems went away.

[boomslack]

my '86 535i was doing this a lot. nice strong pull until suddenly it wasn't at all, felt like no fuel available.

My mechanic and I thought it was the exhaust leak, so we changed that. Then thought it was pump, so we replaced the in-tank and outboard pump. When he was working on something he snapped off a vacuum line by accident, replaced that. it fixed ~most~ of the high-RPM struggling. Maybe the vapor didn't get through the vacuum because the line might be clogged instead of leaking? I'm not 100% sure on what I'm talking about, I just know that fixing a single vacuum line on mine solved 85% of my high-rpm bogging.

[Brayden2544]

Thanks for help guys. I've checked all the vacuum lines very thoroughly, luckily it's really easy on an M30, not like BMW engines of today that have seemingly hundreds.

The injectors may very well need to be cleaned professionally, since they might be coming out anyway I'm going to throw in some LiquiMoly injector cleaner and see if it's helps. I know you can potentially cause issues running injector cleaner but if they're most likely coming out again anyway and I have a new fuel filter I don't see any harm. Worst case scenario it helps a little so I know it's the injectors, best case scenario it fixes the issue.

Waiting for transmission fluid so I won't be able to check for a couple of days, if anyone else has any other potential causes let me know!

Thanks!

[Brayden2544]

Well long time since the update, I tried new injectors from eBay and also had the originals taken out and cleaned.

My only other thoughts now are the FPR or the vacuum line to the FPR being bad even though a smoke test detected no leaks.

I've ordered a fuel pressure tester, we'll see if that tells me anything. Any other insight is appreciated.

[tn535i]

Well this is old. In that high rpm full throttle range the computer goes open loop and at least you can rule out the O2 sensor. However you could monitor the O2 sensor voltage and it might give you a clue. In open loop I think it needs a clean AFM signal and another suspect could be solder joints or other solid state device in the DME breaking down. The duty cycle for the injectors will run higher at high output in higher gears than in 1 or 2 for sure where load is much less and engine revs faster.

fwiw I've chased some of these problems and if you have access to something like a dual trace scope meter it's great. Without that I've actually found my old basic volt meter with fixed ranges and a needle like a speedometer comes in handy. Wire it up to whatever you suspect and watch it's output like you would a gauge. You might be able to see it leading (cause) and other times lagging (response). It should work on O2, AFM, Injectors and crank sensors for example.

[Brayden2544]

Thanks for the input. I checked fuel pressure today and I'm thinking my FPR is bad. Instead of the 43.5 I should be seeing at idle, it's closer to 38/39. I clamped off the fuel return line from the FPR and the pressure would spike to much higher PSI's. When I unplug the vacuum line to the FPR the gauge read at about 45 PSI. Is it common for FPR's to still behave normally just a couple of PSI below spec when they go bad? Trying to rule out that my fuel pressure gauge is reading low since it's brand new and I haven't used it on anything else to verify its accuracy.

The pump and fuel filter are less than 5,000 miles old and were replaced with new Bosch components. The in-tank was replaced by PO supposedly within the last 20k miles so if it's fuel related it would have to be the FPR.

[tn535i]

It sounds like the FPR is doing what it is supposed to do which is increase pressure a few psi when there is little or no vacuum. 3-4 psi low could just be the gauge you are using so it's really hard to say. It should probably jump up to 80 ish when you clamp the return line. Did you register that high, if even briefly ?

I'm not sure a new FPR will fix it and you may just get the same readings and experience with a new one.

You say the PO replaced the in tank pump. Have you experienced the problem with a full tank of gas? Let's say the in tank pump wasn't installed 100% and it's sucking air below some point... that will certainly cause problems as air bubbles make it into the injector. Normally a little air would pass across the rail but at WOT maybe the air bubbles are going into the injectors a bit. I think we had someone finally track a problem like this down to that. The in tank pump connections are iffy with old parts and especially when the pump is kaput it might let air get sucked in at lower fuel levels.

[Brayden2544]

I see no difference when the car is full vs when it is empty.

I do have a leak when I put more than 10 gallons into the tank so there is definitely an issue somewhere with the tank but I think it's just a hole in the filler neck or seam which will be fixed hopefully in the next week or two with a little welding and POR 15.

I fear you might be right about the gauge just reading low, but I will be turboing the car with micro squirt and the TCD kit once I get these issues sorted so a brand new FPR didn't seem like a bad idea in either case. I should know tomorrow if that helped or not when I install it.