Can a faulty immobilizer make the engine backfire into the air intake. The car starts and ticks over nicely, but as soon as it warms up all hell lets loose when i press the throttle. So far i have fitted new plugs and coils, new fuel pump, new fuel filter, new cold start temp sensor, new Maf sensor, also i have had the Catalytic Converter tested. One thing that was mentioned a few times was the crankshaft position sensor, could it be that. I give up, please help. Rob.

 

Could be an issue with the Fuel Pressure Regulator - if the diaphragm is shot fuel could get sucked in to the inlet manifold. Pull the Vac pipe off the Regulator and see/smell if any fuel is present in the hose.

Also try testing the regulator with a vac pump at the vac inlet and see if it can hold vacuum.

 

Hi Brendan, Thank you for getting back to me. We have changed the Fuel Pressure Regulator, in fact there is not much we have not changed. What are your thoughts on the Crankshaft Position sensor. Rob.

 

I consider the Crank Sensor a maintenance item. It's not a question of if it fails, just when. So if you haven't changed that in recent memory (or even if you have) it's worth a shot. Testing it requires specialist equipment (Scope) so substitution is normally the way to go. Having a spare sensor in the boot is no bad thing anyway.

Having said that, I've never heard of a faulty one causing a back fire but who knows - lots of threads on this forum where the sensor caused all sorts of strange driveability issues.

If it fixes your issue let us know and we can add that to the list of CPS failure symptoms.

 

Have you thoroughly checked for a vacuum leak/air leak. Backfiring into the air intake is a sign of a lean mixture. You have replaced most components relating to this including the MAF.
My experience with this type of malfunction is to look at the simple things first & then proceed to the complicated. Does not take much, a slightly split or badly fitting vacuum hose is all it takes.

John Herbert

 

Hi Rob,

It will help us give you the most accurate replies if you tell us the year, model, engine and mileage of your car. It's a good idea to add this to your signature so others don't have to ask.

Have you scanned the car's electronic control modules for any stored diagnostic trouble codes? That may give you some helpful clues.

In addition to a leaking FPR as Brendan mentioned, a common cause of backfires is a leak in the air intake plumbing or vacuum hose, or a problem with the emissions systems like a stuck open carbon canister purge valve or EGR valve.

A weak fuel pump could lead to lean running and backfiring. Have you checked your fuel pressure? It's not convenient on an X300 since there is no Schrader valve.

Intake valves that are not sealing well could allow backfires in the air intake.

While the crankshaft position sensor (CKPS) provides the timing control signal for the engine control module (ECM), and poor timing can cause backfires, I haven't heard of a failing CKPS causing poor timing. The only way it could, as far as I know, is if it was mounted improperly in its bracket. And that would likely cause poor running at all engine temps and speeds.

The fact that it only backfires after it warms up makes me suspect the engine coolant temperature sensor (ECTS), which tells the ECM to apply cold-start fuel enrichment. If it is stuck on a "cold" signal, the ECM will continue to enrich the fueling, leading to overfueling, choking and possibly backfiring. Is this the "cold start temp sensor" you replaced?

Other suspects would be the O2 sensors since they directly affect fueling, and are out of the loop for half a minute or so at cold-starts while their heaters bring them up to operating temps.

To answer your first question, no, I can't think of any mechanism whereby the immobilizer system could cause engine backfires.

Please let us know your car's details and any codes you've scanned.

Cheers,

Don

 

Thanks guys, there,s a lot there for me to consider. Rob.

 

What? No year, model, engine and mileage of your car? Come on, Rob, help us out here!

 

Hi Don, The car is a 1995 X300 Sport, 3.2 Mileage is 107000. I live in a remote part of Wales, difficult to get anyone to help with the mechanics round here, However, there is a guy calling mid week to plug it in, so to speak, as we have not done that yet. I just hope the he can pinpoint the problem with his box of tricks as i don't have a clue what I'm looking for. Regards, Rob.

 

Hi tycapel, where in Wales do you live? I live in Chester and have a code reader if you are near.

 

Hi littleic69, Thank you for that offer, I live on Anglesey, not really that close to you. I am expecting a guy to come round tomorrow to plug the car in. Thank you all the same mate. Rob.

 

No problem. Let us know how you get on.

 

Well i finally got it plugged in and it showed fault code P0413 Secondary Air Injection System Switching Valve 'A' Circuit Open Could somebody please tell me where to find this on the engine. Thank you. Rob.

 

Assuming you have a UK model car, it doesn't have Air Injection. This code is commonly reported on UK cars that never had Air Injection.

 

Hi chaps, thank you very much for all the information. We have managed to stop the car backfiring into the air intake, it turned out to be the temperature sensor on the coolant. This has left us with the engine struggling to rev up properly when it gets warm. We have narrowed it down to one of three things, either faulty fuel injectors, EGR, valve or worse case scenario cracked cylinder head. We cant make out if the engine is not getting enough fuel, or is it choking up by something. Could you please point me in the right direction to find the EGR Valve, bit miffed where it is. Regards, Rob.

 

Hi Rob,

Did you replace the engine coolant temperature sensor (ECTS)? If it fails at a reading that tells the engine control module (ECM) that the coolant is always cold, the ECM will continue to apply cold-start fuel enrichment even after the engine is warm, which causes rich running.

Have you scanned the car for any flagged diagnostic trouble codes (DTCs)? With those symptoms it seems likely there will be at least one code to narrow down your diagnosis.

If you've replaced the ECTS, you can check it for proper operation either with a scanner that can show Live Data, or by checking the resistance across the ECTS terminals when the engine is cold and again when the engine is hot. The correct readings are in the Engine Management manual, which you can download here:

Jaguar Engine Managment Systems AJ6 AJ16 V12


To answer your question about the EGR valve (EGRV), it sits at the top of the engine. Assuming it's the same on the 3.2L as on the 4.0L, it is part 6 in this diagram:



And part 1 in this diagram:




If you could, please add your car's model, year and engine details to your signature line so others don't have to go back in the thread to find that information in order to give you appropriate replies.

Cheers,

Don

 

Hi Don, i have updated my signature as you said. Looking at the picture of the EGRV, i don't seem to have that on my car, well not in that position anyway. We have changed the ECTS, but not test it to see if it is working properly. Not a lot showed on the diagnostics thingy, only P0413, which iv been told i don't have on this car. Thank you for your help, Rob.

 

Hi Rob,

Thinking about your symptoms - does the engine run well until it warms up? If so, it is unlikely the fuel injectors are at fault.

Since your car does not appear to be equipped with EGR, we can rule that out.

Other components whose operation varies with temperature are the oxygen sensors (O2S). The ECM ignores them for a period after a cold start to allow them to heat up to operating temperature. During this warm-up period the ECM operates in a mode called Open Loop, which means that rather than referring to the O2S signals to manage the air-to-fuel ratio (AFR), it uses pre-programmed fuel "maps" to adjust the AFR. Once the warm-up period has elapsed, the ECM switches to Closed Loop operation, in which it actively varies the fueling according to feedback from the O2S.

If I recall correctly, your U.K. cars have only two O2S (in the U.S. we have four). If one or both sensors are stuck at one reading or are not swinging as quickly as they should, the ECM cannot adjust the fueling properly, which can lead to lean or rich running.

You can typically watch the O2S behavior with a scanner that can read Live Data to observe whether both sensors are behaving similarly or if one seems to not change as much as the other.

Cheers,

Don

 

Hi Don, One other thing i have just noticed, which may be relevant to the ECTS, is that, even when the engine is cold, ten seconds after turning on the ignition the fan starts up. The guy with diagnostics is coming back on Wednesday, so i am hoping to test the ECTS and the 02 sensors. Yesterday, before reading your last post, i thought i would change the fuel injectors, i wished i hadn't now, as one of them is leaking, have to pull it all out again. Rob.

 

If I recall correctly, on an X300 at least one cooling fan starts running either immediately upon engine start, or even when the key is turned to position II (ON) without starting the engine. If that is correct, then your fan may not be coming on as soon as it should. Hopefully someone else will either confirm or correct.

Cheers,

Don