LS Swap Troubleshooting

**projectDD** · 12-21-2021, 12:26 AM

About 7 months ago, I purchased this car: https://vorshlag.smugmug.com/Vorshla...Swap/i-fk5NPfr.

The car has an intermittent no start issue. The issue presents in one of two ways: cranks but doesn't fire or fires up, but barely idles and then dies after a few seconds. In the scenario where it does fire up but dies, pressing the gas pedal does nothing. Generally, I can turn the key off, try to start it again, and it will fire right up. Sometimes I have to try 3-4 times.

I drive this car almost every day. I can go for weeks at a time with no issues and then some days it will happed 3+ times. It doesn't happen under any specific circumstances i.e. it will happen when hot, cold, after sitting for days or after sitting for 10 min. However, it does seem to be happening more frequently over time.

I have never been able to replicate the no start condition. However, I have noticed that immediately after having this issue, there is little or no fuel pressure at the rail. Therefore, I believe this is a fuel deliver issue.

I was told by the previous owner that Vorshlag installed a Walbro 255lph fuel pump as part of the swap. He later replaced the pump with a stock m3 pump because the Walbro was too loud. I have run both the stock and Walbro pumps and neither seems to have an impact on the no start issue.

I can't figure out how to troubleshoot this since I can force the failure to occur. I gave up trying to t/s and just started throwing parts at it. Here's what I have replaced(none of these had any affect):
fuel pump (now had the Walbro 255 installed)
fuel filter and pressure regulator(C5 corvette unit)
crank position sensor
cam position sensor
idle air control valve
ignition relay
fuel pump relay

My fuel system is identical to the one shown half way down this thread: https://www.vorshlag.com/forums/foru...-95-m3-ls1-t56. It appears to be stock 8mm lines to and from the fpr. I just noticed today that my fuel pressure is at 62psi instead of 58. I diconnected the return line and the pressure dropped to 58, so I must have a partially clogged return. I will investigate that, but don't believe it has anything to do with my starting issue.

I'm at my wits end and I'm starting to worry about driving the car. I don't want it to leave me stranded somewhere if it finally decides to quit starting for good. Any ideas one what could be causing this? Thanks in advance!

**E36M3LX** · 12-21-2021, 11:32 AM

any codes in the computer? does it have a tune? check all the grounds

**projectDD** · 12-21-2021, 01:57 PM

Originally Posted by E36M3LX

any codes in the computer? does it have a tune? check all the grounds

Thanks, I forgot to mention these those. The computer never stores any fault codes. For a while I was carrying my obd2 scanner in the car so that I could check immediately after the car wouldn't start.

It does appear to have a tune. Someone wrote on the ecu case "Headers only, VATS + rear o2 delete."

I have checked all of the grounds that I can find. From what I see online, it looks like most swap harnesses have grounds on the back of one, or both, cylinder heads. I can't see or feel them due to how tight the motor is to the firewall. I may end up dropping the subframe a few inches to see if that provides enough clearance.

**E36M3LX** · 12-22-2021, 12:53 PM

Originally Posted by projectDD

Thanks, I forgot to mention these those. The computer never stores any fault codes. For a while I was carrying my obd2 scanner in the car so that I could check immediately after the car wouldn't start.

It does appear to have a tune. Someone wrote on the ecu case "Headers only, VATS + rear o2 delete."

I have checked all of the grounds that I can find. From what I see online, it looks like most swap harnesses have grounds on the back of one, or both, cylinder heads. I can't see or feel them due to how tight the motor is to the firewall. I may end up dropping the subframe a few inches to see if that provides enough clearance.

I would start with those three things then. check grounds, try to find out who tuned it(even the pro's screw up sometimes), and try to get some codes out of it. have you tried unplugging some sensors(MAF or MAP, coil packs) to force the computer to show codes? another thing to check would be power to the fuel pump. originally, the bmw dme tells fuel pump to run, is the LS ecu telling the fuel pump to prime? or is it on its own circuit? even the best shops can take short cuts at the end of a build and sacrifice the longevity of things.

**projectDD** · 01-11-2022, 03:40 PM

Just wanted to update this in case anyone else has the same issue. I ended up finding a loose ground wire on the back of the driver's-side head. I tightened the bolt up and have been two weeks without incident. I can't say for sure that this solved my problem(only time will tell), but I am hopeful. In addition to not having a starting issue, my idle has smoothed out quite a bit.