Fuel pressure dropping

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wattsmonkey
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Fuel pressure dropping

Post by wattsmonkey »

I have had to press the sixer into daily driver service until I get the sensible car repaired, which has brought a number of long-standing issues to the fore.

Aug '84 build 635, so running the 018ECU, 018 AFM, brown low impedance injectors and the disc-type air slide valve. No in-tank pump, only main.

One of the issues is the leaking fuel pressure - gauge installed inline before rail indicates pressure dropping to zero overnight. This is, I imagine, part of the difficult starting problem that has hastened the demise of my ignition switch as it cranks for a long time before starting - up to ten "chugs", which seems a lot even with leaking fuel pressure.

FPR replaced, pressure rises when vacuum hose removed, so assuming all okay (should be for £240 from dealer)
Injectors (refurbished professionally) removed, fuel pump energised, no visible leaks whatsoever from injectors proper, or from cold start injector (ah! that's why it's a CSi!)
All hoses checked for tightness.
More in hope than expectation, one-way valve installed after pump. Pump is brand new Bosch unit.
Pressure reading just below 40psi when idling
Pressure jumps straight up to 40psi when cranking.
No smell of fuel from rail, sender, pump...
Fuel filter definitely installed the right way around!

Any thoughts gratefully received.

This is due for the long-planned engine swap (M90 bottom end, B34 head/Schrick cam OR B35 highline with Schrick going straight in), so the above didn't really matter (until it did).

Many thanks,

Rob
"Most of it necessary; all of it enjoyable." LJKS
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nick88highline
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Re: Fuel pressure dropping

Post by nick88highline »

Are you expecting the residual fuel pressure to be maintained overnight? I wouldn't expect full pressure to be maintained for more than 20-30 mins and then gradually fall to zero after a few hours depending on temperature.
I'm lucky enough to have an in-car fuel pressure gauge so I can monitor this all the time. My car does not hold pressure for more than a few hours and certainly overnight would lose ALL pressure. I think a fuel pressure gauge is essential in a fuel injected car :lol:
Cranking the engine over that many times before starting is good because it helps prime the oil system! 8-[
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wattsmonkey
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Re: Fuel pressure dropping

Post by wattsmonkey »

Thank you Nick,

Yes - I understood it should stay above 2 bar overnight and drop over a longer period. This is internet wisdom, of course! You've reassured me knowing this is the right side of normal.

Cheers,

Rob
"Most of it necessary; all of it enjoyable." LJKS
'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
plip1953
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Re: Fuel pressure dropping

Post by plip1953 »

As you say yourself, irrespective of the number of "chugs", fuel pressure at the correct level seems to be present almost instantly. In which case doesn't that seem to rule out low fuel pressure as your issue?

When "chugging", does the engine try to "catch" at all earlier in the process, or is it simply an extended number of chugs and then all systems go? What if you run the car for a few mins, switch off, and then restart pretty much straight away? Or if you try to restart after an interval of, say, 30 mins?

Presumably the rate of "chugging" is strong? Nice fresh battery?

Were you provided with a flow report when you had the injectors cleaned? If so, is there anything to compare against? And do the injectors definitely match what the ecu is expecting to see?

Have you tried doing a cold start, stopping before the engine catches, and then removing a plug to see if fuel is present? Could it even be over-fuelling?

Grasping at straws, at a cold start, any chance the ecu is not seeing the need to add fuel to compensate for the lower temps?
Phil
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wattsmonkey
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Re: Fuel pressure dropping

Post by wattsmonkey »

Thank you Phil, it appears to be a number of little problems adding up to a bigger one! Replacement ignition switch has made things more comfortable, too.

Fuel pressure regulator problem #1, air slide valve #2, both appeared to be working normally in isolation, but change to less-worn spares improved things. As likeliest culprits they warranted replacement.
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olympia57
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Re: Fuel pressure dropping

Post by olympia57 »

Does the pump not have a NRV inside it ?
Immediate 40psi when cranking may not necessarily mean you have 40psi of fuel.
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wattsmonkey
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Re: Fuel pressure dropping

Post by wattsmonkey »

It certainly should have, Don - it's new, but something's up.

Starting much faster, but still running rough until "cleared throat". Am now in parts replacement bingo territory, but all doable.

Have installed NRV to rule that out.
Valve clearances done today
new Bosch W8DC plugs today
new ignition switch
new fpr - now holding pressure much longer.
new air slide valve - but pretty sure it's an M635 one.
swapped out csv
installed Alpina front apron (!)
swapped out spare AFM

Of the above, the Alpina front apron had the most pronounced effect on driving enjoyment.

I'll get there - thanks all for confirming pressure dropping is normal - that was the bit that was sending me down a rabbit hole.
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'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
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sharkfan
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Re: Fuel pressure dropping

Post by sharkfan »

Is it driving ok once started or is the problem purely the cold start length of cranking?
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nick88highline
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Re: Fuel pressure dropping

Post by nick88highline »

Just to add some detail to my post above about fuel pressure dropping. Here are some readings I took on a relatively warm day (21oC):

2.7 bar at shutdown, ie idle pressure
3.0 bar after 20 mins
2.4 bar after 1 hour
1.1 bar after 2 hours
0.0 bar after 7 hours

Not sure exactly when it went to zero, but I suspect 3-4 hours. I consider these to be "normal", ie no leaks.
Might be helpful to someone at some point!
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Re: Fuel pressure dropping

Post by Pod »

wattsmonkey wrote: Sun May 21, 2023 8:05 pm Of the above, the Alpina front apron had the most pronounced effect on driving enjoyment.
I’m intrigued, Rob - are you saying that you felt happier with the apron fitted, or that it improved the handling?
wattsmonkey
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Re: Fuel pressure dropping

Post by wattsmonkey »

Thank you Nick, that is extremely reassuring. I am hopeful that I'm in the situation of having fixed a problem that may have been there beforehand with fuel pressure/delivery and am now dealing with bodges to bypass valve/afm adjustment done to mask rough running in the past. Now starts well, but runs rough until warm.

Kevin - yes, runs fine once warm, there is a slight flat spot 1,500-2,000 which I suspect might be afm related, but is still really nice to drive.

I really need some specs for the 018 AFM, especially what the resistance should start from for the Air Temp Sensor - it goes down when hot air applied, but I'm guilty of "a little bit of knowledge is a dangerous thing" in making assumptions that all's well! It is very much lower than the spare, reading 300 from ambient temp, whilst spare reads over 1,000.

Pod - it's the former, apron delivered mojo boost! I wasn't even sure I was going to keep the 628 bumper, but I'm really glad now. It just about clears speedbumps, but I'll be living in fear of kerbs for the duration.
apron.jpg
apron.jpg (142.23 KiB) Viewed 962 times
"Most of it necessary; all of it enjoyable." LJKS
'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
plip1953
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Re: Fuel pressure dropping

Post by plip1953 »

As you say, a little knowledge is a dangerous thing! I reckon I I've learned quite a lot in recent months about how thermistor temp measuring devices work. My experience has come from Honda engines where the coolant temp sensor appeared to be reporting 10C higher temps at around 90C and this apparent "overheating" was playing havoc with what the ecu did to boost settings. Anyway, there were two fixes, the first being to change the sensor for one that the ecu was actually programmed to work with (rather obvious), but the other was to add a 20p resistor inline in one of the wires to the sensor. In my case the required resistor size was only 50 ohms, but pretty much any rating of resistor is available.

To arrive at the end position I remove the sensor it and put it in both iced water and boiling water and measure the resistances at those two, and you can plot intermediate ones too to see how resistance drops sharply as temp rises. And of course what you're aiming for is match the resistance of the sensor you have to what it needs to be (at a particular temp). Give my a call if any of this might be on the right lines for you.
Phil
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