EVAP starting strangeness

AZmoto

Well-Known Member
I have a brand new ATK LNF engine with a new engine harness. The first few times I started the engine it cranked right up and idled fine. The other day when I went to put it on the trailer for the DMV inspection it wouldn't start. After about the 3rd or 4th time cranking I would get a mild backfire. If I give it a shot of starting fluid in the air filter it starts and runs fine. It will also start again on its own once it's warmed up.

I pulled the codes and got one for fuel tank pressure voltage and another for the EVAP system. After some experimenting I found that the engine will start from cold with the EVAP unplugged. The line on the EVAP solenoid is capped but I've tried it both ways and it doesn't make a difference. I've disabled all of the EVAP functions in HPTuners but that only seems to affect the actual CEL and not whether it runs the self diagnostics.

So right now I can only cold start the car with the EVAP unplugged. I can plug it back in after it starts and it's okay. It seems like the ECM is changing it's starting parameters based on whether it detects the EVAP solenoid. Has anyone else run into this?
 

Ross

Goblin Guru
I wonder if it is just the cold weather that is causing you problems. Maybe tune the tables at the lower temperatures? Is unplugging the EVAP is just a way of changing the air fuel mixture?
 

Gtstorey

Goblin Guru
Unplugging the Evap Solenoid disables the Long Term Fuel Trims on the LNF. I really don't remember if the LTFT goes back to zero after ignition off, I'll check a log this evening. My first guess is the MAF needs additional tuning although I'm a little unclear how much it uses the MAF on startup. There are so many tables that can't be accessed by HP Tuner for the LNF.

What does your LTFT+STFT look like compared to the MAF frequency?

I also don't think I get a code for "fuel tank pressure voltage" so you might be having fuel pressure issues.

What did you do about the 4 bar MAP sensor the ATK intake had on it (at least mine did)?

There are people on HP Tuners than ran with the EVAP solenoid unplugged and just let the STFT handle adjustments. Since the best anyone can tell the LTFT is just a single value, it isn't as sophisticated as most vehicles that have at least a load/rpm table with various adjustments. There has been some discussion that the LTFT does as much harm as good.
 

Gtstorey

Goblin Guru
I also want to point out that when I had a problem starting on mine, It seemed to start only after unplugging and then re-plugging one of the relays in the fuse box. Checked it probably 10 times and it worked every time. Finally it didn't work and I traced it to a loose wire connection on the fuse box that just happened to reconnect after wiggling the fuse box to unplug and re-plug that relay.
 

AZmoto

Well-Known Member
Well today it wouldn't start right away even with the EVAP unplugged so the EVAP may be just coincidental. It may just be a tuning issue that's getting worse. I'm running the MAP sensors off of the donor. I also have Opel injectors and E85. The tune is stock other than the parameter changes for the injectors and the fuel.
 

Gtstorey

Goblin Guru
I checked my logs last night but it was hard to tell whether the LTFT was resetting at ignition off since most of my logs are from after a tune update. I "think" it carries the LTFT over on an ignition off since I found a few logs that had it going from start up.

If it was me, I would chase down the fuel tank pressure voltage problem to start with and see if that helps. Send me the P#### and I'll post the service manual steps to diagnose it.
 

Ross

Goblin Guru
The purpose of the Long Term Fuel Trims (LTFT) is to keep the data long term, ie: after the ignition is off, and between drives. So you are right, the trims carry over.
 

Ross

Goblin Guru
Oh. Well the LTFT work that way on the LSJ. Not sure on the LNF. Kind of pitty you guys, as there isn't much info on LNF tuning.
 
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