Loss of power under hard acceleration

[pepsih]

Hey guys I have a 1986 635csi and im having an issue under "hard" acceleration above 3 3.5k rpm,
so basically if i press the gas pedal very gently i can go to 4krpm without loss of power hover assuming i press almost full throttle it loses power at 3k and above.
i replaced the in tank fuel pump, filter and pump, still have the issue

what should i like into next?

thank you

[1986series6]

I would do a smoke test to find a vacuum leak if any.

https://www.amazon.com/Honey-Keeper-...60435187&psc=1

I use this and connect a hose to the spout, connected
to the tip of a water bottle with bottom cut away to
fit over ATM, duck tape to seal. I use Jack Daniels smoked wood to get
that whisky flavor while I'm at it.

[RSheiman]

Prob a vacuum issue as Mr Daniels said but make sure your FPR is functioning properly. Also have you ever cleaned or repositioned the arm of the AFM?

[DesktopDave]

I'm voting for worn tracks on the AFM too, but I'd also want to see what's going on with the throttle position sensor. If you have an automatic, that TPS has a lot to do with timing. I'd also be testing both position & speed sensors on the bell housing. Although it's a less likely troublemaker, it's a "while I'm in there" thing to do. Once you've isolated those causes we can go with the "less likely" problems.

Where's the Hi-Performance Motronic troubleshooting guide?!? Wait, they changed their name to HPSI. Here's their updated guide. Well worth printing out a copy for your garage library. You have Motronic Basic, also known as 1.0. Ignore the rest of it.

I'm quite partial to a nice applewood smoker myself, but I'm certainly not going to disagree with the charred white oak! ;-)

[pepsih]

The TPS is good, the value of the speed and position sensor is the same on both: terminal 1-2 approx 950ohm between 1-3 and 2-3 "open loop"
i've check based on that picture...
testing_ref_sensors_216.png
can this be the issue?


Also im not really sure if that could give more informations but I have a feeling that the car temp affect it also, Im saying that because yesterday during my test drive when fairly cold the engine was able to go above 4k rpm, and when the engine is warm around 2-3k it starts to loose power.
Just FYI the car is low miles (43k) and was sitting for years in a storage, i mean a lot of things were done like replacing fuel pump and filters, the tank is not 100% but fairly clean.


Also i just checked the Coolant temp sensor (CTS), I MEAN i hope I did it right, check the picture below....
WhatsApp Image 2022-11-24 at 10.17.03 AM.jpeg

so i got:
5120 ohm@ 45f (7.2c)
1400 ohm @ 86f (30c)
772 ohm @ 110F (45c)
303 ohm @ 136F ( 58C)

I FOUND ON another forum that bosch specs are : 3792ohm @ 10c, 1174@40c, 834@50c, 596@60c
also the link says: " If you have a 528e with Motronic 1.1 or a 535i you are looking for a slightly different set of values. With a coolant temperature of 12-16F (-9C to -11C) resistance between the two terminals should be 8200-10500 ohms. At a coolant temperature of 66F-70F (19C-21C) it should be 2200-2700 ohms. When the engine is fully warmed up the coolant temperature should be 174F-178F (79C-81C) and you should have 300-360 ohms of resistance."


look like im way off, no???


and here is the video of the issue

https://youtube.com/shorts/QQT7uuBv0oY (nevermind the anti-lock light, the sensor is on the way )

[RSheiman]

Last I looked, CTS was available and cheap.....like under 20 bucks. Just swap it out, 2min.

[DesktopDave]

The TPS is good, the value of the speed and position sensor is the same on both: terminal 1-2 approx 950ohm between 1-3 and 2-3 "open loop"
i've check based on that picture...
testing_ref_sensors_216.png
can this be the issue?

Those readings look OK to me, every DMM seems to have a different readout for a very high resistance/open circuit. Mine says "infinity", which I think is cool.

Also im not really sure if that could give more informations but I have a feeling that the car temp affect it also, Im saying that because yesterday during my test drive when fairly cold the engine was able to go above 4k rpm, and when the engine is warm around 2-3k it starts to loose power.
Just FYI the car is low miles (43k) and was sitting for years in a storage, i mean a lot of things were done like replacing fuel pump and filters, the tank is not 100% but fairly clean.


Also i just checked the Coolant temp sensor (CTS), I MEAN i hope I did it right, check the picture below....
WhatsApp Image 2022-11-24 at 10.17.03 AM.jpeg

so i got:
5120 ohm@ 45f (7.2c)
1400 ohm @ 86f (30c)
772 ohm @ 110F (45c)
303 ohm @ 136F ( 58C)

I FOUND ON another forum that bosch specs are : 3792ohm @ 10c, 1174@40c, 834@50c, 596@60c
also the link says: " If you have a 528e with Motronic 1.1 or a 535i you are looking for a slightly different set of values. With a coolant temperature of 12-16F (-9C to -11C) resistance between the two terminals should be 8200-10500 ohms. At a coolant temperature of 66F-70F (19C-21C) it should be 2200-2700 ohms. When the engine is fully warmed up the coolant temperature should be 174F-178F (79C-81C) and you should have 300-360 ohms of resistance."


look like im way off, no???


and here is the video of the issue

https://youtube.com/shorts/QQT7uuBv0oY (nevermind the anti-lock light, the sensor is on the way )

I think your CTS is good, the values are working just fine and you tested the correct one. FYI, the other plugs are for the temp gauge sender (brown plug), and the vac-operated thermotime switch (two vac lines & green plastic disk).

Take a little time to go through that HPSI Motronic Troubleshooting Guide I mentioned above. It's an exceptional piece of work, it really helps to cut down on guessing and eliminate possible causes. I know it seems like a huge job, but once you get moving it'll be done in an hour or two at the most. Your car uses Motronic 1.0 Basic. The other cars you've listed use an upgraded version of Motronic. Those versions (1.1 & 1.3) aren't compatible with your older Motronic 1.0. It's easy to tell the difference though. If your car has a smooth crank pulley, it's Motronic 1.0 or earlier. If it has a toothed crank pulley, it's Motronic 1.1 or 1.3. It's relatively easy to upgrade 1.0 cars to the later 1.3.

Since you've eliminated the CTS and TPS, I'd spend a little time trying to track down vac leaks and testing the air flow meter (AFM). They last a long time, very durable, but are notorious for causing throttle hesitation when the internal resistor wears down a little.

[Bert Poliakoff]

If I am not mistaken, your car should have Motronic 1.1. The 1.0. as far as I know, went out the window on the post 82 cars. Easiest way to tell is your ECU should in 002. My euro is 1.0 and my understanding is 1.0 was on the model year 81 Euros.. It was the first year Motronic was used.

[pepsih]

So Today i found some time to work more on the car...
i did the fuel pressure test, here are my results...
with engine off pressure 45 psi
with engine idling, approx 38 psi.
when i removed the vaccum line the pressure rise to about 45psi with engine on.

so to me the pressure was a little low, so i decided to check the fuel pressure regulator, the manual says :
" If the fuel pressure is too low it could be either a bad fuel pressure regulator or a bad fuel pump. It's pretty easy to figure out which one. Simply perform an engine off fuel pressure check as described above. Slowly pinch off the fuel pressure regulator's return line, if the pressure doesn't rise you have a bad pump. If it does rise you have a bad regulator. "

so when i slowly pinched the return line i did not see the difference, in order to see any difference i really had to squeeze the return line.... then the pressure rised by a lot, so i decided to replace the regulator.... however after replacing it i decided to the the same test and i had similar results. my intank fuel pump is brand new(aftermarket) , i have a new fuel filter and the fuel pump is less than a year old.
i did not test drove the car yet!

so im not really sure if that fixed my problem ... i doubt it but we will see.

[RSheiman]

I have never driven without an in-tank pump but from what I have read, it's contribution is such that it should not be your problem. Also, go on Ebay and buy a spare ECU which is cheap money. Always good to have one so when issues arise, you can swap and exclude an ECU issue out of the gate.

[pepsih]

Idk but i feel like my fuel tank is not clean and clogged the new fuel pump... so here is a little background, this car has been sitting for X amount of years before i bought it, before the person sold it to me. he brought it to a mechanic and he "drained the tank replaced filter flush the lines" i remember during the test drive the almost died on me because of a similar issue... and also the tank never showed "empty" so one day i run out of fuel but as soon as i filled up full tank the car seemed to be working much better. Last time when i removed the intank fuel pump i noticed some debris inside the tank, i vaccumed as much as i could but looks like the bad has already been done to the car. so my next step will be to remove that tank clean it, reinstall it, get a new fuel pump, and we go from there

[DesktopDave]

Definitely a good idea! A clogged filter sock in the tank will cause flow problems. I also test for flow when I'm doing the pressure tests, you should get at least a liter per minute. I think the spec is two liters/min. A healthy pump should be able to push nearly 80psi if you dead-head it with a pressure gauge. I'm assuming you'll also replace the main fuel filter. I also like to check the check valve (?) on the pressure pump while I have it all apart.

Your car should have two pumps, a "lift" pump in the tank and a pressure pump under the car. They work as a team, the lift pump providing flow so the pressure pump can keep the rail pressurized. Our early Motronics have no way of sensing fuel pressure, so they'll assume it's always correct. The DME can compensate slightly if the O2 sensor is still working, but only in a very limited way. I'd guess it'd retard the timing and hold the injectors open longer. The car will typically run OK with a failed lift pump, but it'll run out of steam pretty quickly with high fuel demands.

[sienayr]

I highly highly recommend using this por15 system for your fuel tank, simply cleaning it won’t fix the root cause and you’ll end up right back in the same situation and may end up with another ruined fuel pump. This will take care of the problem for good. I used this last year after tracing similar symptoms to a corroded fuel tank and I’ve had no issues since.

https://por15.com/products/fuel-tank-repair-kit

[RSheiman]

If your tank has ANY issue, I too recommend the POR-15 system. Stuff is amazing. Not as difficult to work with as one my think based on the directions.