Won't start when it's hot., no start Options

[pcdarks]

I have a stock 1972 914 that has been in the family since 1976. In that time it has always had an intermittent starting issue. When the car is shut off when it's hot or has just sat in the sun it win't start. It doesn't do it every time and It will turn over but won't start. Mt mother had the car in the 70s-90s and when she would come to visit It would do this in the sun so she got in the habit of parking it under a tree and would never have a starting problem. I have a suspicion it's the computer. Any thoughts? thanks.

[914werke]

Need more info...
OE 1.7L? FI?

[Bleyseng]

Nah, it can’t be the ECU (computer) but try installing a hot start relay per Mark Henry’s post.

[pcdarks]

Need more info...
OE 1.7L? FI?

yes

[pcdarks]

Nah, it can’t be the ECU (computer) but try installing a hot start relay per Mark Henry’s post.

can you send a link to it? thanks

[Superhawk996]

Start by ensuring the ground between body and transmission is low resistance. As measured with a DMM it should measure ZERO ohm. A starter draws a very large current.

Voltage drop = Current Draw x Resistance it travels across.
Example: 12V = 50A x 0.24 ohms

With only 50 Amps potentially drawn by the starter a 0.24 ohm ground resistance would drop the whole 12 volts across that high resistance ground junction.

The stock system as designed functions properly if the wiring is sound.

Although a hot start relay is not out to the question for a car with really screwed up wiring that isn't easily fixable, I've seen a good deal of cars that had a secondary hot start relay that failed to solve starting problems.

To repeat the point, If you have a bad ground anywhere between battery and the transmission housing, a hot start relay will not solve the problem.

Another key point to check is to make sure the joint between the starter motor and the transmission bell housing is clean. Corrosion in this joint will lead to high resistance but doesn't show up when measuring resistance from battery to body, or body to transmission housing.

Start by replacing battery cables (both positive and negative) AND the ground strap between the body and the transmission (3 cables total) before you resort to a hot start relay. I've personally solved hot start on multiple vehicles with this alone.

[pcdarks]

Start by ensuring the ground between body and transmission is low resistance. As measured with a DMM it should measure ZERO ohm. A starter draws a very large current.

Voltage drop = Current Draw x Resistance it travels across.
Example: 12V = 50A x 0.24 ohms

With only 50 Amps potentially drawn by the starter a 0.24 ohm ground resistance would drop the whole 12 volts across that high resistance ground junction.

The stock system as designed functions properly if the wiring is sound.

Although a hot start relay is not out to the question for a car with really screwed up wiring that isn't easily fixable, I've seen a good deal of cars that had a secondary hot start relay that failed to solve starting problems.

To repeat the point, If you have a bad ground anywhere between battery and the transmission housing, a hot start relay will not solve the problem.

Another key point to check is to make sure the joint between the starter motor and the transmission bell housing is clean. Corrosion in this joint will lead to high resistance but doesn't show up when measuring resistance from battery to body, or body to transmission housing.

Start by replacing battery cables (both positive and negative) AND the ground strap between the body and the transmission (3 cables total) before you resort to a hot start relay. I've personally solved hot start on multiple vehicles with this alone.

It will turn over all day long but it won't start so I doubt if bad connections are the problem as the starter works fine.

[cdn914]

Replace the head temperature sensor.

[bbrock]

So after it cools down, it starts okay? Do you know where the fuel pump is located? These cars were notorious for vapor lock and there was a factory recall to move the fuel pump from the engine compartment to under the steering rack to solve this issue. This sounds a lot like vapor lock to me but I hesitate to mention it because people who know these cars a lot better than I do have responded and not mentioned it. Now I'm curious about why. (IMG:style_emoticons/default/confused24.gif)

[Superhawk996]

It will turn over all day long but it won't start so I doubt if bad connections are the problem as the starter works fine.

Oops. My bad. Back to hooked on Phonics. (IMG:style_emoticons/default/icon8.gif)

Back to basics for engine not running but cranks.

Engine needs 3 things to run:
1) Fuel
2) Spark
3) Compression

You can likely disregard compression since it runs otherwise.

Before randomly replacing things you need to narrow down the other two.

When it won't start but cranks. Pull a spark plug and check whether it is sparking? If so, this is not the problem. If it is not sparking (when grounded properly - ask for details if you don't know how to do this) you can now focus your efforts on this area.

When it wont start but cranks - use a (short) shot of starting fluid down the throttle body. Does it cough / sputter / or run for a second? If so fuel system is your problem. If it is in the fuel system, there is a published troubleshooting guide for both L-jet and D-jet. Follow the published troubleshooting procedure. Per Bret's suggestion it could be vapor lock but this "trick" will help you firm up that theory.

Fixing vehicles is not about guess work but a logical progression of determining where the problem is.

[pcdarks]

Replace the head temperature sensor.

Part ordered

[pcdarks]

I replaced the fuel pump recently and it is located in a very small area under the car on the passenger side just in front of the wheel. There are no heat sources near it. Is this the normal place for the pump?

[Superhawk996]

If the fuel pump has been relocated to front, vapor lock is a whole lot less likely. Not impossible but not likely.

[pcdarks]

I'll try checking spark and fuel .

[bbrock]

I replaced the fuel pump recently and it is located in a very small area under the car on the passenger side just in front of the wheel. There are no heat sources near it. Is this the normal place for the pump?

Front or rear wheel? I'm having a hard time envisioning where it is. Doesn't sound like a stock location but if it has been moved somewhere it won't heat soak on a hot day, I agree with @Superhawk996 that you can move vapor lock down on the list of possibilities.

BTW, there is a kind of fun work around to vapor lock that doesn't involve moving the pump. I used it with great success on my first (1970) 914 as it was prone to vapor lock on 90+ days. You pop the cap off the fuel pump relay and drill a hole just large enough to fit the clicker cap from a ball point pin through. The flange on the bottom of the clicker holds it in place. Put the relay cap back on and now you have a manual fuel pump override button. If the car vapor locks, you turn the key to the run position and then press the button for 20 seconds or so to circulate cool fuel through the system. Then start the car. Used it many times during hot Kansas summers and it worked every time. (IMG:style_emoticons/default/biggrin.gif)

[Superhawk996]

Your on your way.

Let us know if you get stuck! (IMG:style_emoticons/default/beerchug.gif)

[Superhawk996]

BTW, there is a kind of fun work around to vapor lock that doesn't involve moving the pump. I used it with great success on my first (1970) 914 as it was prone to vapor lock on 90+ days. You pop the cap off the fuel pump relay and drill a hole just large enough to fit the clicker cap from a ball point pin through. The flange on the bottom of the clicker holds it in place. Put the relay cap back on and now you have a manual fuel pump override button. If the car vapor locks, you turn the key to the run position and then press the button for 20 seconds or so to circulate cool fuel through the system. Then start the car. Used it many times during hot Kansas summers and it worked every time. (IMG:style_emoticons/default/biggrin.gif)

That is a new one for me. Never heard of that. Innovative. (IMG:style_emoticons/default/beerchug.gif)

[bbrock]

BTW, there is a kind of fun work around to vapor lock that doesn't involve moving the pump. I used it with great success on my first (1970) 914 as it was prone to vapor lock on 90+ days. You pop the cap off the fuel pump relay and drill a hole just large enough to fit the clicker cap from a ball point pin through. The flange on the bottom of the clicker holds it in place. Put the relay cap back on and now you have a manual fuel pump override button. If the car vapor locks, you turn the key to the run position and then press the button for 20 seconds or so to circulate cool fuel through the system. Then start the car. Used it many times during hot Kansas summers and it worked every time. (IMG:style_emoticons/default/biggrin.gif)

That is a new one for me. Never heard of that. Innovative. (IMG:style_emoticons/default/beerchug.gif)

I think it was a tech tip in a very old issue of Mid-Engine Views.

[pcdarks]

fuel pump is mounted passenger side in front of the rear wheel.

[Spoke]

If you spray starting fluid in the intake does it catch for a moment? This will isolate fuel from electrics.