Redline RPM immediatley on startup - 1.8 L-jet Options

[GreenLantern]

Finally getting my 1973 1.8 L-Jet back together after years due to a full tear down. Engine's rebuilt, Trans Rebuilt, rust repair etc.

I've been troubleshooting startup issues and I am at the point where after starting the car it immediately climbs up to red line at which point I shut it off.

Trying to figure out what's hooked up wrong or failed. I tried plugging vacuum lines and running them how I believe they should run and it had zero impact. So I really don't think this is a vacuum issue but an L-jet issue.

With the air flow sensor top off I can see the flapper door go from closed to wide open on coinciding with the RPM climb on startup.

So what is causing this?

Air flow sensor -
Temperature Sensor - 1800 Ohms at 85 F - Seems correct
Fuel pressure regulator -
Full Throttle switch - Maybe stuck open?
Cold Start Valve -
Thermo-Time Switch -
Auxillary Air Regulator -
Deceleration Valve - I don't think this is coming into play at all

[wonkipop]

Finally getting my 1973 1.8 L-Jet back together after years due to a full tear down. Engine's rebuilt, Trans Rebuilt, rust repair etc.

I've been troubleshooting startup issues and I am at the point where after starting the car it immediately climbs up to red line at which point I shut it off.

Trying to figure out what's hooked up wrong or failed. I tried plugging vacuum lines and running them how I believe they should run and it had zero impact. So I really don't think this is a vacuum issue but an L-jet issue.

With the air flow sensor top off I can see the flapper door go from closed to wide open on coinciding with the RPM climb on startup.

So what is causing this?

Air flow sensor -
Temperature Sensor - 1800 Ohms at 85 F - Seems correct
Fuel pressure regulator -
Full Throttle switch - Maybe stuck open?
Cold Start Valve -
Thermo-Time Switch -
Auxillary Air Regulator -
Deceleration Valve - I don't think this is coming into play at all

hmmm.

well its able to pull the air from somewhere into the inlet plenum if its moving the AFM flap.
ie something is acting like a throttle plate/ if its not the throttle plate open.
so its not like thinking of a standard vacuum leak.
L jet won't usually run at all with a standard vac leak.
rather its something open internal to the system thats letting it draw the air via a "legit" route.

two culprits i would think of straightway to trouble shoot -
AAV itself. its wide open? not shutting.
and it could be the decel valve also stuck open.
either one of them provides a direct passage for the inlet air which will activate the AFM.

try plugging the decel valve and aav off and see what happens?
do them one at a time.
disconnect them where they plug into the boot below the AFM.
plug the boot inlet and plug the two hoses.


EDIT
to assist. where i would start looking at first.
ps - when you say its a 73 with L jet i take it you mean its a 74 built in calendar year 73.
no 73 stock had L jet.

[Robarabian]

KISS principle question..... (Keep it simple stupid)

Is the clutch pushed in when you start it? Could your throttle and clutch cables be interacting. That happened to me once....

If not, then something mechanical certainly.

[Rob-O]

I wouldn’t think the decel valve or the AAV would feed enough air to go to redline. My guess would be that something is holding the throttle body plate open. So my suggestion would be to disconnect the throttle cable. This will help determine if the cable being wrapped around something in the tunnel, or the accelerator pedal mechanism itself is causing the issue. Next I’d make sure the air bleed valve on the throttle body is closed completely. Then open it again to maybe 1 full turn. That should be enough to let the car idle but not rev high. Then I’d look at the springs on the throttle body itself. Are they in good shape? If you actuate the throttle valve by hand on the throttle body does it return/close? Lastly, is the supplemental spring connected to the throttle body and then the cooling tin at the BACK of the engine? I feel like many see the small hole on the plenum and think the return spring connects to the throttle body and the plenum. It doesn’t. It’s a safety mechanism to return the throttle plate closed if the return spring on the throttle body breaks.

If the return spring on the throttle body itself is there, unbroken and installed correctly the throttle body should close. If there is a spring from the throttle body to the back tin of the motor then the throttle body should definitely close.

Has the throttle body been disassembled or messed with?

[GreenLantern]

SOLVED (IMG:style_emoticons/default/smilie_pokal.gif)
Thanks for the great reply's and your suggestions. I had only driven this car a few times before tearing it down so it is new to me. Of course that was 5 years ago and 100s of hours of work later. Everything is a learning experience.

Car is a 1974 with a 1.8 L-jet. I think it was originally a 2.0 and a previous owner stole the 2.0 for a race car they had and swapped in the 1.8.

I have been reading like crazy and found a great guide for the L-jet on the VW Vanagon which is very similar and it helped me gain a basic understanding of how the system works. When I saw the return spring on the inlet plenum in this guide I had high hopes I just reversed the throttle but quickly realized it is opposite in that application vs the 914.

So after lots of head scratching, disassembly, multimeter measurements, tweaking and finally re-assembly it dawned on me what was wrong.

The return spring was weak on the throttle body and didn't close it all the way without the helper spring and I clamped the cable before putting on the helper spring the first go around.

When inspecting the springs I adjusted the spring tension on the throttle body and when I re-assembled I realized 'closed' is now a a very different point where it clamps to the throttle cable.
Embarrassed by my stupidity but thank you. Rob-O you nailed it, Robarabian KISS wins out , and wonkipop you got me started thinking though the problem and pointed me in the right direction.

Now onto timing, drive shafts and transmission adjustments. Who knows, I might even get to drive this thing before the snow falls at this rate.

[wonkipop]

(IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)

the old vw mirroring trick. (IMG:style_emoticons/default/beerchug.gif)

[TRP]

As a fellow 1.8 LJet owner, I'm happy to see this one get resolved quickly. (IMG:style_emoticons/default/beerchug.gif)

[Rob-O]

No reason to be embarrassed! We all learn sitting on the shoulders of those who came before us. I learned (almost) everything I know about L-Jet from the members here. Particularly Clay Perrine.

I’m happy you have it running!

You say the return spring was weak? I’ve found that the spring has more than enough force to return the throttle plate to a closed position. That helper spring is there in case the return spring on the throttle body breaks. The helper spring is there to prevent exactly the issue you were experiencing…a runaway throttle situation. You may want to check that spring. If I remember correctly it can be installed incorrectly and cause it to seem weak.

[wonkipop]

No reason to be embarrassed! We all learn sitting on the shoulders of those who came before us. I learned (almost) everything I know about L-Jet from the members here. Particularly Clay Perrine.

I’m happy you have it running!

You say the return spring was weak? I’ve found that the spring has more than enough force to return the throttle plate to a closed position. That helper spring is there in case the return spring on the throttle body breaks. The helper spring is there to prevent exactly the issue you were experiencing…a runaway throttle situation. You may want to check that spring. If I remember correctly it can be installed incorrectly and cause it to seem weak.

(IMG:style_emoticons/default/biggrin.gif)

thats right. there is a special place reserved for that helper spring in 914 mirror world on the very back of the tin in the opposite direction to the VW parallel universe. that little hole in the intake plenum seam joint that serves a purpose on ya rear engined VW sedans can seem so enticing.

(IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)