weird high idle with nearly empty tank Options

[dax1969]

Hi,

I have a 74 2.0 FI and car idles, after warming up, at a normal 850/900 rpm

Yesterday however I had something weird ... In the morning (really cold outside) everything was normal (higher idle on start up, normal idle when warmed up)
but on the way back the idle remained at 1200 rpm. I immediately thought vacuumleak.

I needed gas as I was running very very low on fuel (warning light came on, never saw it before (IMG:style_emoticons/default/biggrin.gif) ) ... fyi, my tank is shiny and clean as I cleaned and coated the tank last year.

After filling up the idle went back to normal... Drove the car again next day, same...all normal.

Can low on fuel cause a higher idle ?

weird

cheers
Dax

[Lockwodo]

I wouldn't think so, more likely a coincidence with some FI glitch. I'd just drive it awhile and see.

[Superhawk996]

A theory:

An empty tank contains much more fuel vapor than a full tank. That is a fact.

Wondering if you are somehow pulling vapor directly through a worn out charcoal canister that is saturated and it’s going directly into the intake and causing a higher idle - maybe in conjunction with an AAR that isn’t 100% closed.

Mostly wondering out loud - no proof this is it.

Alternately you have a minor vacuum leak or AAR didn’t close 100% and the next drive cycle heat loosened thing up and vacuum leak sealed or AAR then closed 100%?

Just throwing ideas out there. Would need to troubleshoot deeper to know for sure.

[ChrisFoley]

There's a little partially crimped vent tube in the filler neck. Sometimes that tube will get blocked and the tank will develop suction - enough to affect fuel pump flow.

[emerygt350]

I agree with Chris, it could be a lean condition from suction.

but that AAR is always a worthy suspect. Particularly if the fast idle just happened to match your cold fast idle. Although I would expect it to be higher since the engine is warm.

[76-914]

AFAIK that is normal. While I've had my '76 down for body, paint and general restoration I've kept ~ 1gal of gas in the tank during this period. Needless to say it's run out several times and I just add another gallon. When it is just about out and at idle it will pick up the rpms. It really fooled me yesterday after I had gone through the fuel system, adjusted FP, air adjustment on TB and idle control on ECU. Got it to idle @1000rpm with no surge or hunting and was quite pleased with myself. one minute later the RPM jumped to 1200 and I started fiddling with everything again to no avail. Fuel pressure had dropped from ~39psi to ~36psi. (IMG:style_emoticons/default/WTF.gif) Then it dawned on me to look into the tank and lo & behold it was running on fumes. Added another gallon and Bob's your uncle. (IMG:style_emoticons/default/beerchug.gif)

[emerygt350]

That is really interesting! Now I have an excuse to find out if my low fuel light still works...

[Lockwodo]

..and a reason to keep a few gallons of gas in the tank... (IMG:style_emoticons/default/driving.gif)

[wonkipop]

just shows that a 914 can actually empty its tank bone dry and the pump can scavenge and deliver all the way to that last bit at just sufficient pressure.
not many cars can do that these days with internal tank fuel pumps.
you usually just get a splutter and they conk out.

i remember my dads old victa 2 stroke lawnmower did exactly this high rev thing as the last gasp when you switched it off. because you turned off the fuel tap and it then emptied the carby bowl to shut down. it did not have a kill switch.

[ejm]

Most likely you had icing in the throttle body which forms when cruising and keeps the throttle from closing all the way. When you stopped for gas heat from the engine melted it. If you were pulling a vacuum on the tank you would probably have noticed something when you removed the cap. The very early 1.7 had intake air preheating but the system was dropped on 914's although other applications of the type 4 engine retained it.

[dax1969]

Thank you all for the feedback... I'll keep an eye on it but two things are for sure :

* my warning light for low fuel works !

* I'll fill up the gas tank earlier (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)


Not in the mood of testing it again (IMG:style_emoticons/default/biggrin.gif)


Greetings
Dax