Warm Start stumble/multiple attempts... where to start looking?

[petethepug]

I found out the hard way that the crank position sensor / CPS can go bad and not throw a code. It had hard starts, slow starts, no starts & stumbling out starts until it finally gave a P0335 and the tach died.

Just as soon as the engine is restarted, symptoms are gone. Two weeks later, same. Ironically the cps on our 02 Audi allroad did the same until it refused to start.

 

[Charlie207]

Yes, after shutoff the heatsoak causes it to go up. Think it's normal.

My fuel pressure rises to about 84psi after every shutoff. Watched it via my Torque App 5 times yesterday, and did a screen record on a couple of them, but Imgur only allows 60 second video clip uploads. Either way, the pressure slowly rises to ~84psi over the course of about 2 minutes.

As for holding the gas-pedal to the floor prior to turning the key, yes, that prevents the engine from starting. After I let off the accelerator and turned the key again, it fired right up.

 

[Charlie207]

check valve in the fuel pump, high under hood Temps and no return line. it is what it is.

It was 12 degrees yesterday. I can't imagine there were high underhood-temps.

Where is the fuel pressure sensor located?

 

[Charlie207]

NM...this video explains it. Looks miserable.

 

[Doubeleive]

It was 12 degrees yesterday. I can't imagine there were high underhood-temps.

Where is the fuel pressure sensor located?

I would guess that outside temps really do not have much to do with it, provided it had been driven and was up to normal operating temperature first. heat rises you def do want too touch a hot engine even it is -40 below. the fuel rail is right there at the top where the most heat is and as soon as you re-start it within a couple seconds the fuel pressure is going to return to normal. you can watch the gauge literally drop like a rock back to 44psi
if the fuel remains high after a re-start then you may have a issue.

 

[Charlie207]

I would guess that outside temps really do not have much to do with it, provided it had been driven and was up to normal operating temperature first. heat rises you def do want too touch a hot engine even it is -40 below. the fuel rail is right there at the top where the most heat is and as soon as you re-start it within a couple seconds the fuel pressure is going to return to normal. you can watch the gauge literally drop like a rock back to 44psi
if the fuel remains high after a re-start then you may have a issue.

Nope, fuel pressure drops instantly to 44ish, like you describe.

Might be worth firing the parts cannon at a new CPS, to see if that helps. Or, maybe a new TPS too.

On a side note, would a new TPS help with sluggish* throttle response.

 

[mikez71]

I found out the hard way that the crank position sensor / CPS can go bad and not throw a code. It had hard starts, slow starts, no starts & stumbling out starts until it finally gave a P0335 and the tach died.

Just as soon as the engine is restarted, symptoms are gone. Two weeks later, same. Ironically the cps on our 02 Audi allroad did the same until it refused to start.

Same thing happened with my '94 BMW 318ti, bad crank pos sensor.
Started off as a couple hard cranks before intermittent starting, and finally failing on me while driving.
Old CPS wiring insulation was disintegrating into gummy twizzler like material when I took it off..

BUT since Strutaeng's flood clear start method works.. Guess he maybe right!

Something similar I read on hptuners forum.. GenV's use fuel trims to determine transmission pressures..
And as the injectors age and leak, fuel trims go down, causing trans pressure to drop and start causing problems.
Dunno how true..

 

[hagar]

Did your tuner reply back? I have been a tuner for 20 years, and it is mega common to need to tweak the hot start with a cam. You generally have a guessed base adjustment on cranking VE to compensate for the change in cranking efficiency from a cam. You start with a rough learned value based on the application, then it sometimes needs slight adjustment to dial in. It's never an issue when cold because all the extra air density has no problem lighting too much cranking fuel.. it becomes a problem when the engine temp is up.
It could be adjusted too much, or too little, just takes a quick couple changes to nail down.

 

[Doubeleive]

Nope, fuel pressure drops instantly to 44ish, like you describe.

Might be worth firing the parts cannon at a new CPS, to see if that helps. Or, maybe a new TPS too.

On a side note, would a new TPS help with sluggish* throttle response.

I am not saying it is your issue, but I personally wouldn't bother with replacing just a tps I would just buy a new throttle body, if it was from a money standpoint I would just buy a used one and roll the dice.
*sluggish might not be the throttle body, that's when I would take a closer look at the maf and see if it is causing shift issue's
if it was jerking and shifting funky that definitely points me to the maf IF the transmission is known to be good
if it "coughs" so to say when giving it pedal for no other reason that has been the throttle body in my experience OR if it idles weirdly low to almost shutting off.
in any case I would try and look at the scanner data and see if anything rears it's head before firing the parts cannon.
check for any vacuum leaks, check that the intake is on securely, especially around the maf and throttle body any un-metered air is going to cause problems
I have seen the maf cause a multitude of problems, my experience has been that at or around or after 160k the maf can be a problem. 160k on a maf is eol in my book

 

[mikez71]

Did your tuner reply back? I have been a tuner for 20 years, and it is mega common to need to tweak the hot start with a cam. You generally have a guessed base adjustment on cranking VE to compensate for the change in cranking efficiency from a cam. You start with a rough learned value based on the application, then it sometimes needs slight adjustment to dial in. It's never an issue when cold because all the extra air density has no problem lighting too much cranking fuel.. it becomes a problem when the engine temp is up.
It could be adjusted too much, or too little, just takes a quick couple changes to nail down.

Hagar! Long time no talk (since I had that KR issue)

Is this the adjustment, or somewhere else?