Hot Start Problems

[Yooks]

Hi all, hope all are well. I can almost guarantee that I will not be the first to post this problem but here goes. I have a US 1971 w113 with e ignition Bought from SLS in Germany. About a year 18 months ago I had the car restored, but ever since it has a real problem with starting hot, starting cold is ok, checked plugs, leads new injectors, timing spot on, Co spot on, not FIP. Can anybody point me and my very compitent mechanic in the right direction please.
Please Please Please anyone
Thanks in advance

[Pawel66]

There are numerous posts on this forum about hot start issues. If you use search function, you will find most of them in an instant.

In essence it looks like 90% of the hot start problems are related to low fuel supply.

First point is to understand that during hot start your engine is in extremely lean condition, so if your Fuel injection Pump is set on the lean side, the hot start issue will be more pronounced.

As per the user manual you should hot start with gas pedal depressed at least one third.

You should make sure your cold start device works ok, meaning fuel is injected through Cold Start Valve during cranking also during hot start. A lot of Members here have a separate additional switch installed to prompt that fuel injection on hot start.

But then the most important question is about fuel supply overall. The simple test is to check the fuel flow\volume at the end of the return fuel line with fuel pump switched on - you should get 0.8 to 1.0L in 15 seconds. Anything below 0.6 L will not be enough.

If the fuel flow is lower than I stated above - you have a filter in the tank, filter in the inlet of the fuel pump and the main fuel filter. All must be cleaned. Return line may be clogged as well and it has to be clear for fuel supply to be adequate.

It is a good practice to install additional flow through filter between the tank and the fuel pump.

It sometimes happens that owners find it necessary to replace the fuel tank with a new one - the old ones have the tendency to produce lots of dirt clogging the filters.

This is what i would start with.

[Yooks]

Thanks for the reply, my mechanic had checked that the fuel pressure is flowing ok and it is he even bought a new pressure guage ti illiminate a false reading. I think we can dismiss fuel pressure. We have also checked the csv and the tts too. But still no luck

[mrfatboy]

When exactly are you having the hot start issue? For example, at operating temp can you turn off engine then turn it immediately on with no problem?

Does the hot start issue manifest itself after 5, 10, 15 ...60 minutes etc after you turned off engine?

[Pawel66]

Again: fuel pressure measurement e.g. at the main filter is something different than the fuel flow measurement at the end of the return line. Check the fuel flow at the end of the return line. It is a very simple test. There are several stories here on clogged return line being the culprit for engine fuel starvation. it may be a bit counterintuitive, but you need to eliminate it.

Another trick is to activate the CSV manually when cranking to imitate the work of the cold start system (needed also with the hot start).

To clarify: I am not a mechanic, but I have been through this topic from A to Z.

[mrfatboy]

Another tip during hot start is depressing the gas pedal 1/3 to 1/2 way during cranking. I believe this is even stated in the manual.

But the questions above by Pawel and myself will help diagnose the issue.

[Yooks]

Thanks for the reply, my mechanic had checked that the fuel pressure is flowing ok and it is he even bought a new pressure guage ti illiminate a false reading. I think we can dismiss fuel pressure. We have also checked the csv and the tts too. But still no luck

When exactly are you having the hot start issue? For example, at operating temp can you turn off engine then turn it immediately on with no problem?

Does the hot start issue manifest itself after 5, 10, 15 ...60 minutes etc after you turned off engine?

It Manifests sometimes after about 20 mins

[mrfatboy]

After 20 minutes or so would make sense to me. At this time the engine has cooled a bit. This most likely places the WRD in the “lean dip” period of the air/fuel mixture.

See the interactive FIPi app in the tech manual under the injection pump section for more information.

Pawel has already suggested a possible solution to this problem but adding a manual override switch to the CSV.

The other solution is adjusting the oval shims in the WRD so the “lean dip” is not so pronounced. This could take many iterations to get right. There is plenty of info on the subject.

[Pawel66]

Another one is to replace the TTS with one that squirts fuel for a bit longer time. Some research here would be required on this forum and tech manual. In my case it was a clogged filter and then I improved hot start by installing a modern Bosch TTS.

But no remedy would work if you do not have the fuel flow right  - which is confirmed by the test I suggested.

[Yooks]

Thank you once again for your help. I will again check the fuel pressure and the return pipe but am almost cirtain that this is not the issue, I will keep you posted

[kampala]

Yooks

You are probably correct that fuel may not be your problem.  The reason Pawell is emphasizing it might be because you have confirmed “pressure” but not confirmed “volume” and you can have one without the other.   Many have had the problem you describe. 

Good luck.

[Pawel66]

Kampala, thank you for this point. What if you do have a flow that builds pressure at the measuring point, but there is no return flow? We know clogged return is causing fuel starvation.

[Shvegel]

You are almost certainly experiencing vapor lock.  Essentially, what is happening is the fuel in the fuel lines is boiling and forcing the liquid fuel either out through the injectors or back into the fuel pump.  When you try to start your car you have to refill the lines so the injectors are not spraying fuel.

When you say the car was just restored 2 things come to mind one is the fuel injectors were replated or got dirt in them. Have them tested. Of particular importance is what is the pressure after 30 minutes. They need to hold at least 30 pounds to keep the fuel from boiling If your injectors are new have them tested anyway. There has been a rash of bad ones.

The other end of the system is much more robust. You have the later ball style check valves (delivery valves) and they are very trouble free. However if the top fittings of the pump were removed for plating they may not have changed the white plastic seals under the delivery fittings.  I would change them.  Without typing a novel as to why these should be changed with the pump on it’s side and maximum cleanliness should be observed and the fitting tightened to specification.