Interesting engine starting problem

[HeliosE21]

I’ve been helping out a fellow forum member with a starting problem with his 1981 320i (M10)

When dead cold – Car will not start (engine fires but stalls immediately). Once the fuel pump relay is removed and a jumper is used (bridging terminals 15 & 87 at fuel pump relay socket) engine fires and runs. After a minute the engine is turned off and the fuel pump relay is reinstalled, engine turns over and runs fine. Attempts to start the car again with relay is successful for only a couple of times immediately thereafter or after a short drive. Further, when the engine is shut off for apprx.10 minutes or longer, (engine cold or warm), it would not start with the fuel pump relay. However, once the car is started with the Jumper again and the relay reinstalled, engine starts again and runs fine.

In summary, relay does not start the car all the time. However, car can be started with relay anytime (engine cold or warm) after it has initially started with a jumper.

Below are Items that have been checked

• Swapped out fuel pump relays – does not seem to solve the problem.
• Cold start system seems to be working (cold valve sprays when engine is starting)
• Wiring and connectivity at coil (terminal 1) to 31b on fuel pump relay is good.
• Wiring and connectivity from 31 at fuel pump relay to Lamba relay is good.
• There is 12v across terminal 31 and 31b to operate fuel pump relay.
• Cleaned the ground connections
• Installed a new blue vacuum hose from the tee near the valve cover to the underside of the intake manifold
• Sprayed a bit of carb cleaner on the fuel injector seals and found a small variance at idle.

Any though suggestions as to what might be the cause? Thanks in advance

[Bill5Star]

Fuel accumulator? Designed to avoid vapor lock.

[GDAus]

Hi ...

Are the contacts on the fuel pump fuse and the fuse holder clean and tight?

A bad connection on the fuse is a common problem. Try spinning the fuel pump fuse in the fuse holder while squeezing the holder contacts to put pressure on the contact areas of the fuse and holder and then see if it starts from cold.

A bit of oxidation on the fuse contacts could be enough to cause enough resistance when cold to stop the relay working ... but when the system is run with a jumper instead of the relay, the higher current of the pumps etc causes enough heat across the contacts allowing enough current flow for the relays to work until it all cools down again.

Cheers

[YoungSensei]

When my m10 in my e21 was having trouble re-starting after being shut off shortly, I replaced my fuel accumulator and external fuel pump, solved my issues. I had the same symptoms as you, and was also able to re-start the car by jumping the fuel pump relay. Try the fuel accumulator first if you want to save money, otherwise go for both parts.

IMG_3534.JPG

[DavidF]

Agree, failed accumulator makes restarting difficult. But, usually, my car would start cold with the failed accumulator...it was more a warm starting issue.

[Bill5Star]

YoungSensei -

That's the way to do it. Did you buy the Bosch pump and smaller accumulator that's still available?

[YoungSensei]

YoungSensei -

That's the way to do it. Did you buy the Bosch pump and smaller accumulator that's still available?

Got it from autohausaz, managed to dig up the order, was purchased in December of 2021. I believe I got the metal fuel piping from ecstuning.
fuelparts.png

[HeliosE21]

Replacing the main and transfer pump due to perhaps being "weak" and possibly the fuel accumulator was discussed as solutions to the starting problem. The parts cost are still reasonable. We were just trying to get that "Low Hanging and inexpensive fruit" first especially because when the car does run, it runs pretty good when driving around the block.

GDAus' recommendations for careful attention to the electric connections around the fuses and relay socket have already been addressed although perhaps giving it a second go around and swapping out to a new fuse might help to keep the fuel pump relay "on" during start up.

Very thankful for everyone's time in reading the post and offering solutions. Will circle back regarding this peculiar problem and what course of action will be taken and of course still open to any other possible solutions.

[gsumzzz]

Original 1981 ignition switch? You’d be surprised. Pull it and inspect.

[Clio320i]

Last year I had a similar issue. Turned out to be the fuse (I, an idiot, did not know there even was a fuse). Symptoms initially were classic bad FPR. I, being not *always* an idiot, keep a FPR relay jumper wire in the car.. Was at first able to jumper the relay with the wire, then after that it would start normally with the relay back in, both immediately after jumpering and for several starts thereafter. Then one day, the trick didn't work, and no amount of jumpering or fiddling with relays would work. I actually had to leave the car at a friend's place and come back the next day after researching and discovering that there is indeed a fuse, and that fuse was indeed very old and corroded and it just finally couldn't make any connection anymore. So, exactly as GDAus described upthread. When I removed the fuse it crumbled apart in my fingers.

Ignition switches can indeed get crappy, but the car in question is starting with the key and the jumper reliably, so I'd say that's higher-hanging fruit at this stage.

I have also had the main fuel pump die. It was immediate with no known warning symptoms (and stranded me on the uphill part of a 4-lane bridge with no median nor shoulder crossing the middle of Portland. Luckily I had a functioning hazard switch back then and was able to slowly roll backwards, causing traffic annoyance I'm sure, and into a parking lot or something).

In-tank pumps can be inop and start/run is still fine. In my experience a failed in-tank pump presents as stuttering or even stalling when taking a hard corner to the right with less than 1/4 tank of gas.

I just checked to listen for the humming or buzzing sound that should accompany the key being turned to the ON position before starting. As I understand that's the Frequency Modulation Valve or Pulse Valve. With the relay, it doesn't make the sound every time in my car, and when it does happen it's only for a couple seconds: https://www.youtube.com/shorts/TCM9Dm3hbCU

Then I did it again with the jumper wire: https://www.youtube.com/shorts/RxIboq7pzQk - it's every time, and it stays on as long as the key is in the ON position.

So I guess I'm with GDAus on checking the fuse area yet again. I saw a recent video making social media rounds of a guy advocating for taking a dremel and a small soft wire wheel to all your fuse holder connections. Thought that was a nice maintenance thing to do.

EDIT: another thought: if there isn't a good replacement fuse handy, borrow the one from the cigar lighter/power antenna - that's the one that is all the way at the exact opposite end of the fuse box, closest to the brake fluid reservoir.