635CSi hesitates

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scarab
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635CSi hesitates

Post by scarab »

I’m fighting hesitation issue on my manual ‘88 635CSi. It happens while cruising, it looks like this engine dies literally for a half of a second, the same moment battery charge light flashes and then it goes back to normal. It accelerates smoothly, the power is fine, no issues here at all. Usually it happens while driving with a constant speed, barely while accelerating. I guess it’s just random, it just happens once a while. Doesn’t seem to be depending on the engine temperature. I’ve already replaced main relay, fuel pump relay (both OEM BMW), cleaned idle regulating valve, checked throttle position sensor, reflowed DME board, cleaned DME connector, cleaned sensor connectors under the hood. Flash code shows 1444 - no errors, no check engine light as well. Do you guys have any idea how to troubleshoot it? :-k
Last edited by scarab on Mon Jul 06, 2020 6:27 pm, edited 1 time in total.
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Re: 635CSi hesitates

Post by plip1953 »

Maybe an bad earth strap or something like that?
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Re: 635CSi hesitates

Post by scarab »

Grounding strap seems to be ok. I’ve already cleaned battery connectors and earth strap connection with the chassis.
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Re: 635CSi hesitates

Post by plip1953 »

You need far more competent people than me to comment, but the fact that the ignition light illuminates has got to be significant. But you also say that the dash "flashes". Can you elaborate on that please?
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Re: 635CSi hesitates

Post by scarab »

I mean that the second engine hesitates battery charge light on the instrument cluster illuminates.
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Re: 635CSi hesitates

Post by 86_6series »

The battery light will come on when the engine idle drops (hesitates)
because the alternator is not spinning fast enough to feed the circuits.

I wish I could help, but an intermittent problem like that is very difficult to diagnose.
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Re: 635CSi hesitates

Post by scarab »

Yep, that’s the thing. It’s intermittent and it’s there since I’ve purchased the car. I even thought main relay replacement did the trick as I’ve made like 50 miles drive afterwards and there was no hesitation at all. And yesterday I drove 10 miles and it happened like 15 times. It really sucks and I have no clue how to narrow down the issue, I would like to avoid blind replacing random parts. I’m not sure if that’s a coincidence as I don’t drive my e24 daily but it’s possible it happens more frequently after filling in the tank, but that wouldn’t make sense.
Could MAF sensor cause that kind of hesitation? It looks like someone was opening it before.
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Re: 635CSi hesitates

Post by plip1953 »

86_6series wrote: Mon Jul 06, 2020 11:49 pm The battery light will come on when the engine idle drops (hesitates)
because the alternator is not spinning fast enough to feed the circuits.
But in this instance, while the hesitation is occurring, the engine will surely be rotating at well above idle, and yet the ignition light still comes on.

I guess what I'm looking to establish here is whether what happens is the same of in some way different to what would happen if you briefly turned the ignition off and back on with the key. And if it is any different, how so?
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Re: 635CSi hesitates

Post by 86_6series »

OK, let assume your right that the engine rpm is ok when hesitating but yet the ign lights come on.
That would lead me to think it certainly isn't fuel, which means a momentary ign circuit hiccup.
Checking what could be that hiccup would be tough, because it's momentary meaning you can't test anything
when it's in the hiccup state.
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Re: 635CSi hesitates

Post by bakemono »

I'm trying to imagine a scenario where the battery light comes on with the engine still spinning and it also causes loss of power. If it was just failing alternator brushes or a slipping belt I wouldn't expect the engine to hesitate.

Could it be a bad connection on the back of the alternator? Or a short circuit? If you have a spare alternator I'd try swapping that out, just to rule it out and get a good look at the wiring while you're at it.
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Re: 635CSi hesitates

Post by plip1953 »

plip1953 wrote: Tue Jul 07, 2020 7:46 am
86_6series wrote: Mon Jul 06, 2020 11:49 pm The battery light will come on when the engine idle drops (hesitates)
because the alternator is not spinning fast enough to feed the circuits.


I guess what I'm looking to establish here is whether what happens is the same of in some way different to what would happen if you briefly turned the ignition off and back on with the key. And if it is any different, how so?
On reflection, what I said above isn't correct. Turning the key would not only turn off the engine, but also all the dash lights.

So what seems to be happening here is that the engine is somehow being denied the means to continue running (for a very short time). If it can be presumed that the regular fuel and/or spark delivery components are ok, could the observed symptoms reasonably be attributed to an ecu issue or perhaps a failing crank sensor?
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Re: 635CSi hesitates

Post by scarab »

So I’ve purchased fuel pressure tester
https://www.amazon.com/dp/B07S9ZJPZ5/re ... bFbW74MERA
And checked the transfer pump (the one that sits in the tank). I jumped fuel pump relay, and the pump itself seems to work. I mean it’s able to pump the fuel without any load, to the bucket. Problem is the pressure gauge shows 0, literally the needle doesn’t move at all. So I’m not sure if the pump is shot and it’s not able to provide enough pressure - According to Bentley manual it supposed to produce 0.3 bar - or maybe the fuel pressure gauge is not sensitive enough. For sure the tester itself is fine as I’ve checked the pressure on the rail in my Jeep and it shows 3bar as expected. So now I’m even more confused.
Update:
I’ve connected the gauge to fuel rail, here are the results:
- about 42psi with jumped fuel relay, engine off. So that’s ok
- about 36psi with engine idling, dropping to 32-34 with opened throttle, should be over 40 according to Bentley manual
- about 42psi with engine idling with pressure regulator vacuum hose disconnected
So it looks like there’s a problem with fuel distribution, I guess I need to address transfer pump first. What do you think guys?
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Re: 635CSi hesitates

Post by plip1953 »

scarab wrote: Thu Jul 09, 2020 1:30 am So I’ve purchased fuel pressure tester
https://www.amazon.com/dp/B07S9ZJPZ5/re ... bFbW74MERA
And checked the transfer pump (the one that sits in the tank). I jumped fuel pump relay, and the pump itself seems to work. I mean it’s able to pump the fuel without any load, to the bucket. Problem is the pressure gauge shows 0, literally the needle doesn’t move at all. So I’m not sure if the pump is shot and it’s not able to provide enough pressure - According to Bentley manual it supposed to produce 0.3 bar - or maybe the fuel pressure gauge is not sensitive enough. For sure the tester itself is fine as I’ve checked the pressure on the rail in my Jeep and it shows 3bar as expected. So now I’m even more confused.
Update:
I’ve connected the gauge to fuel rail, here are the results:
- about 42psi with jumped fuel relay, engine off. So that’s ok
- about 36psi with engine idling, dropping to 32-34 with opened throttle, should be over 40 according to Bentley manual
- about 42psi with engine idling with pressure regulator vacuum hose disconnected
So it looks like there’s a problem with fuel distribution, I guess I need to address transfer pump first. What do you think guys?
Sounds like you've probably identified areas that might need attending , although the numbers don't seem too bad to me.

But I really don't think it would explain the hesitation you describe, especially when you say this occurs at cruising when the fuel demand is low. And nor would it explain why the dash lights come on.
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Re: 635CSi hesitates

Post by scarab »

I believe fuel pressure is clearly not right as it should be 3bar, and what I’ve got when the engine is idling is 2.5 bar. But you are correct, it does not explain why it has hiccups when cruising, unless the pump tends to hang from time to time causing engine to die.
I wonder what causes low fuel pressure. Theoretically, from what I’ve read transfer pump has nothing to do with pressure, it’s rather the main pump or fuel regulator. But faulty regulator should cause the pressure to be to high rather than too low, which points me to the main pump. Am I right?
Update, so here’s my theory:
When the engine idles pressure provided by fuel pressure regulator is low (too low in my case).
When the engine runs with higher load fuel regulator gets more vacuum, opens and provides more pressure, may not be enough to the engine to run with full power but enough for it not to hiccup.
When the engine runs at low load, with less vacuum, pressure regulator doesn’t provide enough pressure which causes hiccups.
At this point I believe FPR needs to be replaced in the first place.
And btw once I clamp fuel return hose the pressure raises like a lot which would mean pump provides enough pressure.
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Re: 635CSi hesitates

Post by Ralph in Socal »

Check what the installed regulator is rated for. There are 2.5 Bar regulators

If necessary install a 3.0 regulator and see if that solves your problem

I think your issue is electrical

Ecu, sensors and relays can cause intermittent shutdown but faults there usually requires a restart of the motor to reset the fault

I think your issue is related to basic Ignition wiring as the engine will refire easily when the fault provides a glitch

Check your ignition switch by moving the key a little while the car is running. Remove additional keys and weight from the ignition key as the weight can cause internal movement within the switch that can cause your intermittent issue. Good luck

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Re: 635CSi hesitates

Post by scarab »

My current regulator is stock Bosch, rated for 3 bar.
When the engine hesitates I haven’t noticed any disturbance in any electrical device like radio, blower etc so I think the ignition switch is fine, but I will double check it just in case.
I believe rpm drop during the hiccup causes battery light to come up. I’m not totally positive as hiccups are pretty short and unpredictable but it’s possible the light doesn’t come up every time, but only when the hesitation is long enough.
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Re: 635CSi hesitates

Post by bakemono »

It's normal for fuel pressure to be lower at idle. The regulator is designed to hold pressure constant relative to manifold pressure, that's what the small vacuum hose is for. Manifold pressure is ~.5 bar lower at idle.

If you had a problem with fuel flow it would show up under high load, ie. when flooring the accelerator the car would start bucking.

If the battery light coming on is not definitely linked to the problem, I would check out some other electrical stuff like flywheel sensors and the distributor cap.
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Re: 635CSi hesitates

Post by 86_6series »

Definitely an electrical problem, as I stated above a starved fuel scenario is when you get a slowing down(starving) or bucking.
A momentary hiccup is electrical.

Stop wasting your time with fuel.
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Re: 635CSi hesitates

Post by scarab »

So I’ve replaced fuel pressure regulator with brand new Pierburg. Fuel pressure hasn’t changed, the measurement looks exactly the same as with the old one. I’ve changed spark plugs as I was about to do it anyways, plus I’ve replaced air flow sensor with used one, sold as tested and fully working. The old one was suspicious to me as it was opened before for some reason. Once I’ve changed air flow sensor I’ve disconnected the battery for 15 minutes, just in case. I took it for a ride, gas level was like 1/2. I’ve made about 4 miles and it hesitated once, and it was very short, hardly noticeable. I made it to gas station, added Techron fuel cleaner and refilled the tank with 91 premium. And there we go, it started having hiccups like crazy, it even died couple of times so I had to crank the engine, it hesitated number of times, basically I put like 10 more miles and hardly made it home. So it looks like it has something to do with the fuel, can’t believe it’s a coincidence that it gets worse when the tank is full. No error codes (flashes 1444), no check engine light. What do you guys think? How about fuel tank breather valve? The one which is below the throttle. Seems to be a suspect.
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Re: 635CSi hesitates

Post by plip1953 »

While stationary, if you hold the revs around 2000rpm for 30 secs or more do you get any mis-firing or hesitations? You mentioned in your first post that the hesitation only happened under cruise conditions, but was fine when accelerating. It seems counter-intuitive to me that fuel quality would cause problems when cruising but be fine when load is applied?

I think I would be inclined to put the original afs back on.
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Re: 635CSi hesitates

Post by scarab »

This is what was happening before. I wouldn’t say it’s about fuel quality but quantity in the tank, plus fuel cleaner may have something to do. But last time it definitely got much much worse after I put a full tank of fuel together with fuel system cleaner so it looks like it has something to do with fuel system. Plus this time it seemed to be unrelated to engine load. It was dying even when I was driving on neutral (I’ve got manual gearbox). I think it’s worth trying to replace the purge valve and double check fuel breather system. I’ve done visual inspection and did not find anything suspicious but maybe the valve is shot.
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Re: 635CSi hesitates

Post by RossDinan6 »

Looks to me like you now have 2 issues.

The original was not fuel in my opinion, There is volume in the fuel lines, rail, etc so would not cause a sudden brief loss of power. Your fuel pressure is not far enough out of range. Electrical would, as mentioned previously. You have an intermittent connection somewhere, I suspect, due to the battery indicator. A study of the ETM is in order to help determine if there is link between the light and ignition cut. I won't sit here and rule out a sensor, but that feels unlikely.

As for the new problem, I doubt the purge system is relevant here. That is an emissions system, the fuel should still be flowing (I have not had to mess with mine, modified car, so could easily be wrong). You might want to recheck the fuel pressure and flow rate. Something clearly changed.
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Re: 635CSi hesitates

Post by RossDinan6 »

Oops, DP
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Re: 635CSi hesitates

Post by scarab »

If fuel breather valve is stuck in open position wouldn’t fumes coming from the gas tank, especially when it’s full interfere with intake air measurements?
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Re: 635CSi hesitates

Post by RossDinan6 »

As I said, I have no experience with the evap system so take this as you may. It is there for emissions purposes so I see no reason for it to affect your drive ability. I just do not think it is your problem. I try real hard to not throw parts at a problem until I am pretty sure I need them. My customers usually appreciate that (I don't fix cars for a living, but the point is the same).
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