Wont start when cold etc

We have had trouble in the past where everyone seemed to jump all over me for getting political when I wasn’t and I took offense. But I am back to see what kind of response and help I can get this time. I am always willing to forgive and forget. But depends on the reception I get.
For your information I did get the '86 Jag running etc after sending all of the plugged up ignitors down to Texas for cleaning. Again I have to blame the terrible alcohol fuel mixture that we get. Before alcohol I can never remember fuel problems like we have now. I am sure many of you have experienced your fuel gage float tank level not working. This has become quite a problem with the only answer of to drop the tank.
But not to get into this touchy subject again here is my problem
I have both a '86 & '87 Jag. The '87 has been sitting without driving for a couple of years also. But did not have the trouble like I did with the '86. Got it running!! But then when I try to start it the next day to add new non-alcohol fuel to it, the girl would not start!!
Cold today, & checked everything out. The cold start ignitor sprays just fine! Thus you would think it would start. Wont!! Checked the ignition which sparks fine!! So with both fuel & spark I figure it should start!! But not even one fire!! So I suggested we disconnect the air filter again and open the air vent and see if the pump is working properly. Just Fine! Then to try again with the air valve open and it starts!!! So what is going on??? Put the air cleaner back on and still starts but not on the first revolution. Am going to try tomorrow morning and my belief is that it again will not start with many revolutions of the starter. I am thinking there may be something wrong with the air/fuel mixture at the air intake. Is this just an open & close valve or what is controlling it? Could be something else? Would sure like your help to solve.
For the fuel pump to operate, the valve has to be at least partially open right? But what if the pressure bleeds off which I have not checked. Where is a good place to hook up a pressure gage? Even if the pressure would bleed off, why wouldn’t it start once the pump brought the pressure up. Something is not working properly and not sure at this point how best to solve. Thanks fellow Jag lovers~~

The usual method for attaching the fuel pressure gauge is to remove the cold start injector hose from the fuel rail. Be ready for some spillage of gas…or even quite a spray if the rail is still pressurized. Now attach you pressure gauge to the fuel rail where the CSI hose was.

When you crank the engine on the starter you should see the gauge go up to 36 psi pretty darn fast…within just a few seconds. If it doesn’t, your pump may be weak. Or perhaps a clogged fuel filter.


Primarily, and briefly, the mixture is controlled by how long the fuel injectors are commanded to stay open. The ECU gives the command in accordance with information from the Air Flow Meter as to how much air is coming into the engine.

Beyond that, the mixture is slightly enriched whenever the starter is operated. If the engine is cold, the mixture is significantly enriched as result from information sent to the ECU by the Coolant Temp Sensor. The mixture is also slightly enriched when the throttle is opened widely, with a switch (“Full Load Switch”) on the throttle body giving notice to the ECU

And, as you know, the Cold Start Injector give a jolt of raw fuel to make cold starts easier.

Since the engine started when you opened the air flap in the Air Flow Meter I wonder if the system is over-fueling, and the extra air compensated? Pulling a couple spark plugs and checking for heavy soot or wetness would be a clue, as would heavy/dark exhaust fumes.


Doug I am so glad that you are still communicating with me as you know so much about these vehicles. Don’t want to remove the cold start injector to determine the rail pressure. I will have to find another way. But thank you so much for answering.

Thanks Doug and as my luck would have it, this problem is weird as is every problem I have even encountered in my lifetime. Still wonder why I was born?? Was born breach you know, (90 years ago) and torn up badly but still here and don’t know why? Medical was not too great in those days!! This problem again makes no sense, as of many past troubles I have encountered. But somehow through hook or crook and simply perseverance, I was always somehow able to solve. Read your post. But does not really give me a path\Lee.

With problems like this the path has various stepping-off points… A fuel pressure test is as good as any and where I’d begin, personally.


Pulling a couple spark plugs is any easy step that can give some clues.

Remove the fuel filter and empty the contents into a clean jar. What you see might dictate your next steps. Blow thru the now emptied filter? Much resistance?

Sometimes it’s helpful to determine was isn’t wrong.


Hello Lee - as Doug mentioned, sounds like you are getting more fuel than you are air so therefore not the correct mixture to fire right away - the clue being that you opened up the path for more air and got the engine to fire up and run - how old, or dirty, are the air filters - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 10/28/2020 2158hrs. EDT USA.

Do you mean the flap just after the air filter?
The further it is pushed open (by the air) the longer the injectors stay open, so more fuel. If it starts with it pushed open it is too lean. It is a very precise metering device besides switching the fuel pump on. How did you check the cold start injection? Only remove the hose to the injector for fuel pressure measurement. Else have a look at the temperature sender to the ECU and the ignition coil. We‘ll get along well as long as you focus on the actual problems, just don’t mention the alcohol any more.

Tex interpreted the valve as the throttle which would mean it’s overfueled… by the way, they always need to crank a revolution or two in most scenarios. In your case a few seconds might be okay. Do you crank them with your foot off the gas pedal?

There is always the possibility of not running the engine long enough on start up confusing the ECU?

Manually opening the AFM further while cranking enriches fuelling, Lee - the ECU air flow by the deflection of the flap. The more air the more fuel is needed - and the more air; the greater deflection. Ie, the engine drawing air opens the flap sufficiently to get the mixture correct - and for cold starts the ECU adds fuel by the factors mentioned by Doug. NB! The amount of air going through the AFM is controlled by the throttle position - not by the AFM flap…

Questions. When the engine started (flap manually opened) what was idle rpms - and what happened when you released the flap? Same result with the operations repeated?

Possible faults that may explain symptoms. The AFM has a a dead spot - a resistor board in the AFM is swept by an arm linked to the flap, giving the required flap position to the ECU. Loss of contact will bewilder the ECU - and pushing the flap may move the arm past the dead spot. You can replace the AFM, as a test, using the other car’s AFM?

You can also disconnect and reconnect the 7-pin connector - on the off hand chance that connections may have corroded…

You have an air leak bypassing the AFM; engine gets the air, but it’s not measured by the AFM - lean mixture. Pushing the flap enriches the mixture - and the engine may start. Carefully check the air duct between AFM and the throttle body - a ‘common’ source of air leaks. Generally, manipulating the AFM flap when there is an air leak will increase rpms - if you get mixture right. Excessive flap deflection will overfuel - and the engine will misbehave, if it runs. However, if fuel pressure is too low, less fuel is injected, lean mixture - but increasing flap deflection will again fatten the mixture…

With a cold engine, extra air is delivered past the throttle and idle screw by the AAV. This gives a higher idle for a cold engine - more power to overcome cold engine drag. Without this extra power, the engine cannot idle cold - and an engine that cannot idle will not start. Disconnect air hose from the AAV an inspect the slide position - with a cold engine the slide should be at least half open. If the slide is closed cold - engine gets insufficient air.
With starting problems, the AAV and the throttle gap should be routinely checked; to positively eliminate these factors - even if the problem seems unrelated…:slight_smile:

As an aside; an eventual leak-down of rail fuel pressure is immaterial for fuel pressure; pressure is restored within a couple of seconds when the pump starts.

xj6 85 Sov Europe (UK/NZ)

Or the throttle “butter fly” Valve is not set at just a bit open. Not enough air to fire. Teh manual flap operation adds not only fuel but air. It fires.

Welcome back Lee, we 90 year olds need company. I gotcha beat turned 91 last month.


I actually removed the injector and it had a good spray! Wondering why it would only start when the flap is held open. Something wrong but don’t as yet know why it refuses to start when cold.

Hello Carl. Boy this getting old is not for sissies is it? I sure have run out of steam.

Thanks Frank. Glad you cleared that up in your last sentence. I was wondering about the fuel rail leak down as a problem. So that should not be a starting problem then. Will do more checking today as it warmed up a bit. Has been really cold here.

Try unplugging the coolant temp sensor and seeing if it starts and runs better. A bad temp sensor will throw the mixture off. With it unplugged the ecu goes to a baseline setting that is somewhat akin to a ‘limp home’ mode in modern cars.

Good tip. What should the sensor read in resistance when at approx 40 deg F outisde?

I’m sorry, I don’t have those specs. I’m sure with a little googling or asking here the specs will be found.

CTS test info is here on Mr. Faircloth’s page:



Those specs look good, and agree with the specs I just found in one of my books on the L Jetronic system:
14 degree F = 7,000-12,000 ohms
68 degree F = 2,000-3,000 ohms
122 degree F = 700-1,000 ohms

Thanks so much for this information. Can’t have enough data on hand to troubleshoot.
Lee Noga