A vapor lock issue?, Starting after 30 minutes or so of sitting Options

[boxsterfan]

So on my 74 2.0L D-Jet, my car normally behaves this way on cold start:

1. Starts right up and idles high while cold
2. As it gets warmed up, the idle "hunts" between 0 RPM and 1200 RPM and can occasionally die when pulling up to a stop sign (you might have to blip the throttle).
3. After she warms up, she idles perfect at 900-950 RPM.

The problem:

1. Car is warmed up driving to a store (idling perfect 900-950 RPM)
2. Go into store for 30 minutes
3. Start car (starts for a second but then dies if you didn't give it any gas)
4. Turn key off
5. Turn key right back on and it doesn't want to start (just cranks). But if you give it gas while cranking you can get her to start....almost like push starting the motor and popping clutch).

Note: My fuel pump is in the front. Stainless steel lines in the tunnel.

Am I still experience vapor lock?
Failing CHTS (also related to idle hunting)
Other?

[JawjaPorsche]

Doesn't sound like vapor lock. Vapor locks usually occurs while driving down the road in hot weather. Your pump is in the front too.

When is the last time you had a tune-up? Points and Plugs?

[TheCabinetmaker]

If it's flooding and it starts by holding pedal to floor, you likely have a leaking injector.

[JamesM]

I have a theory on this problem on both d-jet and megasqirt(using d-jet intake parts and sensors) equipped cars.

What I saw on the Megasquirt equipped car (where I can easily check sensor readings) was that the air temp sensor was getting heat soaked after sitting and was reading the intake temp at near 200 deg. The fuel injection is then leaning out the mixture a ton to compensate for what it thinks is thinner air. Cycling the key on and off a few times on the MS car triggers the prime pulse and would throw enough fuel in to get it to start. On the D-jet car I would pump the throttle with the key on a few times to get the accel enrichment to throw some gas in.

Once started it runs rough until enough air has passed over the temp sensor to cool it off, then all is good.

This was all confirmed with temp readings in Megasquirt, my suspicion is that the same issue affects d-jet

[tumamilhem]

I am having a similar problem. My car was gone through thoroughly and all the culprits were checked. Still occasionally having issues:

- Car will occasionally make slight hiccups in power.
- Sometimes when it gets hot, it won't turn over, just crank. If it doesn't start after several tries, I have to sit there a while so I don't drain my battery and to just let it cool.
- Car idles high when cold, low when hot.

Everything has been checked as good or recently replaced:
- Wires good/cleaned
- MPS good
- Decel Valve good
- Distributor good/cleaned
- Plugs replaced
- D-jet injectors good, one replaced with OEM
- Computer good
- Timing good
- Fresh tuneup, fresh oil
- No points

Still having issues. Not as bad as before though.

[boxsterfan]

I have a theory on this problem on both d-jet and megasqirt(using d-jet intake parts and sensors) equipped cars.

What I saw on the Megasquirt equipped car (where I can easily check sensor readings) was that the air temp sensor was getting heat soaked after sitting and was reading the intake temp at near 200 deg. The fuel injection is then leaning out the mixture a ton to compensate for what it thinks is thinner air. Cycling the key on and off a few times on the MS car triggers the prime pulse and would throw enough fuel in to get it to start. On the D-jet car I would pump the throttle with the key on a few times to get the accel enrichment to throw some gas in.

Once started it runs rough until enough air has passed over the temp sensor to cool it off, then all is good.

This was all confirmed with temp readings in Megasquirt, my suspicion is that the same issue affects d-jet

Your observation describes my symptoms perfectly. What can be done?

[boxsterfan]

If it's flooding and it starts by holding pedal to floor, you likely have a leaking injector.

This is a good lead to follow up on. I also have gas smell in the car sometimes. I have replaced all of the hoses for the evap emissions (cannister, expansion tank, in the wheel well, in the engine bay, hoses under the rocker are not cracked/broken).

[Cap'n Krusty]

Listen up! I'm old enough to have worked at a dealership when these cars were new. I did pre-delivery inspections, services, warranty work, and even major engine repairs on these cars. Lotsa test drives. 2 liter 914s ALL did this from the get-go. Every one of 'em. Floor the pedal, let it off as seems prudent. The FI guys at the Porsche training center were well aware of it, and attributed it to what they called "an electronic lie". The rather dumb analog control unit was unable to prioritize signals, and couldn't allow for heat soak, ambient air temps, and many other factors that are a piece 'o cake for next-gen and later systems. That's also why we sometimes had to unplug the TS1 in the plenum in order to make them run right.

The Cap'n

[JawjaPorsche]

Listen up! I'm old enough to have worked at a dealership when these cars were new. I did pre-delivery inspections, services, warranty work, and even major engine repairs on these cars. Lotsa test drives. 2 liter 914s ALL did this from the get-go. Every one of 'em. Floor the pedal, let it off as seems prudent. The FI guys at the Porsche training center were well aware of it, and attributed it to what they called "an electronic lie". The rather dumb analog control unit was unable to prioritize signals, and couldn't allow for heat soak, ambient air temps, and many other factors that are a piece 'o cake for next-gen and later systems. That's also why we sometimes had to unplug the TS1 in the plenum in order to make them run right.

The Cap'n

Just curious, Cap'n. Was this "An Electronic Lie' only related to the 2.0 or did the 1.7 have the same issues? (IMG:style_emoticons/default/popcorn[1].gif)

[Cap'n Krusty]

I don't recall it being a problem with the 1.7s, but we had far more 2 liter cars on the sales lot than 1.7s.

The Cap'n

[boxsterfan]

Listen up! I'm old enough to have worked at a dealership when these cars were new. I did pre-delivery inspections, services, warranty work, and even major engine repairs on these cars. Lotsa test drives. 2 liter 914s ALL did this from the get-go. Every one of 'em. Floor the pedal, let it off as seems prudent. The FI guys at the Porsche training center were well aware of it, and attributed it to what they called "an electronic lie". The rather dumb analog control unit was unable to prioritize signals, and couldn't allow for heat soak, ambient air temps, and many other factors that are a piece 'o cake for next-gen and later systems. That's also why we sometimes had to unplug the TS1 in the plenum in order to make them run right.

The Cap'n

Thank you for the insight o'Krusted one. (IMG:style_emoticons/default/beer3.gif)

I guess one day I'll get this fixed by switching to MegaSquirt or my ninja LH-Jet project. (IMG:style_emoticons/default/ph34r.gif)

[ky914porsche]

Listen up! I'm old enough to have worked at a dealership when these cars were new. I did pre-delivery inspections, services, warranty work, and even major engine repairs on these cars. Lotsa test drives. 2 liter 914s ALL did this from the get-go. Every one of 'em. Floor the pedal, let it off as seems prudent. The FI guys at the Porsche training center were well aware of it, and attributed it to what they called "an electronic lie". The rather dumb analog control unit was unable to prioritize signals, and couldn't allow for heat soak, ambient air temps, and many other factors that are a piece 'o cake for next-gen and later systems. That's also why we sometimes had to unplug the TS1 in the plenum in order to make them run right.

The Cap'n

Thank you for the insight o'Krusted one. (IMG:style_emoticons/default/beer3.gif)

I guess one day I'll get this fixed by switching to MegaSquirt or my ninja LH-Jet project. (IMG:style_emoticons/default/ph34r.gif)

My "new" 1975 914 2.0 has shown this same inconvenient little nuance during hot starts. It is solved by holding down the gas peddle big time during hot startup, but now my gas pedal push rod keeps disconnecting between the gas pedal and the throttle. It has a new push rod with ball to go into pedal and ball to go into throttle, but those 2 little balls are way too finnicky when the gas pedal is aggressively feathered up and down. I've got to find a way to make them stay in place better, because I'm tired of pulling off the side of the road to reconnect the gas pedal rod to the throttle in the floor.

[cary]

Cap'n will have to give you the details.
But all you have to do is push the throttle to the floor and crank it up. Don't ask me what it does. The only thing your doing is telling the ECU your at full throttle. But it seems to work every time.
Primarily in our 2.0L like the Capn said.

[Tom]

From the owner's manual for the 75 2.0 :
Before starting, depress the accelerator pedal fully and keep at full throttle until engine runs.
The reason for this is as follows: when the accelerator pedal is depressed after the key is turned to the ON position, the ECU receives a signal to lengthen the injector pulse, effectively enriching the mixture.
So the engine is warmed up and driven for xx minutes and you go into a store for xx minutes and come back out and it does not want to start. The cylinder heads are aluminum and cool off at a different rate from the cylinders ( usually steel) and while the cylinders are saying- we are hot, give us a lean mixture, the CHT in the cylinder head is saying - I'm cooling off so give me a richer mixture. End result is an over rich mixture until the heads are warmed back up.
Back to depressing the pedal BEFORE turning on the key switch. If you fully depress and hold the pedal while cranking, the ECU does receive the enriching signal. It is similar to an accelerator pump on a carbed car except you can "fool" the ECU by depressing prior to power on. If you depress the pedal before the key switch is turned to ON, the extra rich mixture does not show up.
I corresponded with Brad Anders on this very subject back in 2008/9 and this is what he told me then. I have tried it and sure enough, it works! We talked about installing a secondary circuit that could be switched on manually to overcome this heat soak issue and he said at the time he was working on a solution, just couldn't find the time to get it worked out and tested. Basically one would need to add a parallel resistive circuit with the CHT and switch it on when you get in to start the car and turn it off once started.
Hope this can help with the understanding of our quirky FI system.
Tom

[TheCabinetmaker]

Ky914, replace the gas pedal. It's the socket that gets worn

[swooshdave]

The cylinder heads are aluminum and cool off at a different rate from the cylinders ( usually steel) and while the cylinders are saying- we are hot, give us a lean mixture, the CHT in the cylinder head is saying - I'm cooling off so give me a richer mixture. End result is an over rich mixture until the heads are warmed back up.
Tom

I doubt the heads will cool off that much faster when sitting in a heat soak situation. I could be wrong but has anyone measured the difference? Won't the heads pull heat from the cylinders?

[76-914]

The cylinder heads are aluminum and cool off at a different rate from the cylinders ( usually steel) and while the cylinders are saying- we are hot, give us a lean mixture, the CHT in the cylinder head is saying - I'm cooling off so give me a richer mixture. End result is an over rich mixture until the heads are warmed back up.
Tom

I doubt the heads will cool off that much faster when sitting in a heat soak situation. I could be wrong but has anyone measured the difference? Won't the heads pull heat from the cylinders?

Tom's right. Those fins on the heads dissipate heat whether the engine is running or stopped.

[Tom]

The thermal conductivity of aluminum is about three times that of iron. Basically this means it takes iron three times as long to heat up and three times as long to cool off as aluminum. All of the fins on the heads are there for a reason. The engineers wanted to get the heat gone as soon as possible. Because of this, the existing ECU has an issue with keeping the mixture right for a car that is warmed up, then shut off for a short period, then started back up. The cylinder (iron) is much warmer than the head ( where the sensor is that tells the ECU what mixture changes to make) and would like a kind of lean mixture. The head being much cooler has the CHT send a signal to the ECU requesting a rich mixture. Now you have a kind of flooded condition. I believe the engineers knew this and that is why the owners manual says to fully depress the pedal prior to cranking. Pretty much what we all learned to do on carbed cars when they got flooded!
Tom