Delayed hot start issue

Problem Description

When the car sits for about 30 minutes after a good run, she doesn’t want to start again. Just turning the key and she will fire and then immediately stall out. If trying to give a little gas, she’ll sputter and backfire if I’m not careful with the revs. After fighting with her tapping the gas just enough, she’ll eventually even out. If I try to start her after just a few minutes of sitting, no problem - this only happens after about 30 minutes of sitting after a good run.

Troubleshooting Process

I read through all the hot start threads I could find and started with the TPS check (red TPS for my Jag). All good there… Or so I thought. I’d tested the TPS several times with different voltmeters and all read within specs - when the car was COLD. After making a BBQ run, the thought struck me to test the TPS when hot. So, when I got home, I set the timer to 30 minutes to mimic the “delay” part. Then I tried to start and sure enough she was not happy. So, I pulled out my trusty voltmeter and here are the findings:

  • Red wire: 105.4 mV
  • Yellow wire: 19.1 mV
  • Green wire: 4.88V

Potential Diagnosis

This would lead me to believe that my TPS does not like to sit around hot without running and may need replacing.


Before throwing a new TPS at this, is there any thing else I should be thinking about?


Look at the connectors for the TPS, especially any crimped joints and the connections themselves. They can look good, but be poorly connected, increasing resistance when hot.

I get same thing, after about 30 minutes. But mine doesn’t stall, just idles a bit low for 20 seconds. I replaced TPS, but exact same issue. I even bought a working vacuum switch (one that increases fuel pressure if fuel rail is hot). No help.

I have assumed the fuel rail is hot, and some of the gas is just slightly vaporized? What may help is turning key 2-3 times before starting engine, which will run fuel pump to flush out the fuel rail.

I have measured heat soak after shut off, and our engines tend to peak at about 20-30 minutes, and then start cooling down.

The ‘check-valve’ in the fuel system might be faulty and VAPOR-LOCK could be the result?
The system needs to maintain pressure for a while to prevent ‘boiling’ of the fuel.

The injectors need LIQUID to operate properly, but low fuel pressure can cause the liquid to turn to GAS (vapor) in high temp environments and cause ‘hot-start’ issues.

If the pressure drops too soon/quickly when the fuel pump is shut down, the check valve is suspected.


I added a push button momentary switch under dash to activate a vacuum valve that allows gas to free flow out of the fuel rail by letting the outgoing FPR to open… I haven’t had a really good opportunity to test it in extreme hot weather with a hot engine… but I have tried it sometimes just to avoid any faltering at start-up. I “believe” that it did help… I’m waiting for a 100 degree day here in Iowa and at least a 20 mile trip to test it out… right now, facetiously, it will be a long wait

A year or so ago I installed a new Walbro fuel pump. I was getting terrible hot starts that would stumble the idle pretty bad, especially on hot days sitting in the sun. Found out the pump did not come with a built in check valve. So I installed my own check valve after the pump, and the hot starts got much better, as described above (just a low idle for 20 seconds).

I wonder if our cars are more susceptible because the location of fuel rail, and how large it is. Most of my other cars have fuel rails 1/3rd the size, and not at top of engine.

How does that work? Some sort of bypass to the FPR?

I have always assumed that the outbound FPR raises fuel pressure (closes) under low/no vacuum, which would correspond to heavy engine load / WOT. And that the inbound must behave in the opposite manner.

Am I incorrect? I would appreciate a definitive explanation, I can guess and assume on my own :slight_smile:


yes it opens up the vacuum to the outgoing FPR allowing the fuel to move through quicker and filling fuel rail with cooler gas… I think it’s helping but really will know on a super hot day if that ever happens

I believe my valve let’s it dump vacuum… I have a small gasoline filter on the hose where it dumps just in case. iirc…we are talking about starting… someone else want to comment to verify…???

I am sure Kirbert will jump in here next time he is around, but in the meantime I will try to explain. You are right about the outgoing FPR function. Low vacuum will raise the fuel pressure in the rail. If the thermal vacuum switch is working, hot fuel in the rail will also allow the vacuum in the regulator to be dumped, which will raise fuel pressure, and consequently the boiling point of the fuel. (Hot start improvement)
The incoming regulator controls pressure between the pump and itself, and operates exactly the same way as the second regulator, not opposite. It is basically a damper, and many have removed it with no ill effects. (As Jaguar did in later years.) Bywater has some comments on this system on his site.

Here is the Jaguar explanation as to what the 6.0L V12 does (just one regulator and NO fuel cooler). Notice that the FPR now operates at 3.0 bar.

Unless the engine is running, there is no vacuum. Pressure cannot be increased.

Problem with hot start usually indicates a fuel-pressure/volume issue. Two-seconds priming will normally purge the whole fuel manifold if there is nothing at fault with the remaining of the fuel delivery system.

It sets a lower rail pressure, but how that would correspond to moving fuel quicker I dunno. It’s a positive displacement pump, it’s going to move fuel at the same rate regardless.

Excuse me, it sets a higher rail pressure if the engine is running. If the engine isn’t running, it makes no difference at all.

I’m going to play dumb here and ask: doesn’t the first FPR regulate the need for fuel in the rail and the other FPR regulate the fuel exit so that the rail has a consistent pressure… so if I dump the vac that operates the outgoing regulater wouldn’t that just let gas move into the rail quicker hopefully getting fuel that hasn’t vaporized to move in… maybe I’m way off on this but that’s the way I thought it works. I’m hoping to hear more from others on this.

I will defer to Kirby and others on this, but if by “dumping vacuum” you mean “no vacuum” under that circumstance the regulator is “closed” and rail FP is high. I think your fix works but for the opposite reason you think it does: It forces a higher rail FP which squeezes out the bubbles that are causing vapor lock. Same principal as the factory setup.


Dave, the first FPR controls the pressure in the line between the pump and the first FPR. It has nothing to do with the pressure in the fuel rail. The second FPR controls the pressure in the rail and its action is determined by vacuum acting on its diaphragm.(Engine load.)
If the vacuum is removed from the second FPR, then fuel pressure in the rail increases, and as the pressure increases, so does the boiling point of the fuel, preventing vapor from forming. Same way a radiator cap changes the boiling point of coolant in a pressurized cooling system.
The actual fuel pressure in the rail (and injectors) is constantly changing depending on how hard you are driving.


One thing that can throw folks off in discussions like these is a misconception about vacuum. High engine loads, the engine operating at peak horsepower, etc. does NOT equal high vacuum. The opposite is true. Engine vacuum is highest at slight throttle openings, like idle, or steady state cruise, or coasting down a hill with a throttle closed. Engine vacuum is LOW when the engine is working hard, when the throttle is open wide, etc.

Back in the late 70s and early 80s mpg gauges showed up on dashboards. They are nothing more than a vacuum gauge. Keep the vacuum high for the most economical operation.

Then, if you really want to nerd out, consider this: diesels generate very little vacuum, because most of them don’t have any kind of a throttle butterfly. Throttle is controlled by simply squirting in more fuel. That’s why you probably won’t see a vacuum brake booster on a diesel.

FWIW I am fighting a hot start problem on my car that got way worse after I installed the manual trans. Once I have a meaningful update I will post, but what used to be a die after start, 500 RPM fluctuation with the auto box has turned into a 0 to 1500 RPM fluctuation, and a difficulty keeping running for the first two minutes or so. After that everything is golden. Next step is to get Rene to help me test the factory hot start circuit, to see if it’s working right at all. I have the vacuum version.

The trick of cycling the key on and off three or four times to pressurize the rail using the fuel pump does seem to help, so I think I’m on the right track.

Another bit of evidence, that may or may not be a coincidence…I’ve noticed my hot start issue got a little bit worse over the last year. One thing that I did do a year ago, was paint my fuel rail black, more Por15 to be exact. Perhaps that is not allowing the heat to escape as well?

Next time I remove the fuel rail, I could remove the paint (although it needs something to keep from rusting?), and put a few little heat sinks on it?! :slight_smile:

thanks for that info… funny how things don’t always appear the same to different people… logical thinking can work in many ways… good thing there is this forum

I did not read through the whole thread but the problem reported is exactly what I had on my Mercedes W126 280SE from 1985. Cold start no problem and difficult to stat after stopping some for time when hot.

The problem was the valve preventing the fuel to return to tank , it was leaking so pressure in the rail dropped . I don’t think you even need to have vapour lock to have that problem , the engine needs to turn until the right pressure is built up in the raiil again .

@Pirk , check fuel pressure in the rail during 30 min , see if there is a loss and if so replace fuel pressure valve accordingly.