Brief lean bog after hot start.

[Sub8]

Hi all,

I was wondering if you guys could propose a likely cause of this issue i have on my 81 911SC.

Issue; After every hot start, at some point in the first 2-3mins of driving I will get a lean bogdown or strong hesitation in vehicle accel. Typically happens at higher revs and heavier throttle. Only once and then doesn't do it again. If I back the throttle down slowly the engine recovers.

Background; bought the car at 137K miles with a tired engine and gearbox. Since then full engine rebuild, full refresh of ignition including a retroclassic CDi box. New airbox and all vac parts. Injectors cleaned and flow checked. Fuel pressure set and correct cold and hot. System vacuum checked. 090WUR. Hego operation confirmed as correct. New fuel pump. Throttle switch checked.

As expected the engine runs fantastically, starts on the button hot or cold, is perfect except for this one particular minor issue.

It did this before any of the rebuild / refresh work I've completed over the last 5 years and at all stages in between the various systems were replaced / refreshed.

The only primary items I've not changed are the fuel meter head and the fuel accumulator.

I've not hooked the CIS pressure tester up to be able to read it while driving under this condition but gut feel is a drop in fuel supply pressure.

Could the accumulator be causing this effect somehow?

Anyone come across this issue before?

Many thanks.

[boyt911sc]

Hi James,

Are you still in UK? Your location shows PA. It is quite difficult to diagnose a problem that happens intermittently. Been reading your other post too but can not offer you a good advise to identify the culprit/s. If you want to diagnose this problem, start with the fuel pressures (cold, warm, residual, and system). These will be your baseline data. And this is MUST for any fuel injection troubleshooting.

I would be curious to know the heater resistance of the WUR-090 at room temp. Keep us posted. Thanks.

Tony

[Sub8]

Hi Tony, yes I am home in the UK. Nearly 3 years now. Miss the big country!

And yes fuel pressure, fuel pressure, fuel pressure!

I've tried hot restarts and revving the engine in neutral with my pressure gauge fitted but haven't replicated the issue. I think because the fuel flow demand isn't as high as when driving.

I don't remember the resistance value right now, other than is was correct hot and cold last time I measured it!

[boyt911sc]

James,

Just post the value when you get a chance. You might be surprised if the value has changed over the years. That is the problem with intermittent occurrence. Thanks.

Tony

[Sub8]

Update - drove it today and it was lean mising the entire journey after a hot restart.

It was fine at full throttle at higher engine speeds.

This made me suspicious of the hego circuit.

Sure enough when I got home, by touch / noise the freq value was not buzzing.

Q - should the valve buz with key on engine off or only engine running?

So I'm thinking that the fv is not running from a hot start. Possible relay fail at high temperature?

[Tyson Schmidt]

Replacing the Lambda relay is a no brainer if the frequency valve isn’t working.