Hard to start AJ16 4.0l


Lately the engine has become progressively harder to start, coupled with a very lean mixture and intermittent rough idle. The fuel flow rate was fine but the pressure in the rail was a dismal 10PSI. Last week I changed the fuel pump - pressure is now 45 PSI, the car pulls like a train and the rough idle has disappeared BUT… the hard starting is becoming even worse.

Any suggestions please?


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #2

What’s the fuel pressure regulator situation on a 4.0?

(Robin O'Connor) #3

Fuel enters the fuel rail by the firewall and the reg is by the thermostat housing if thats what you are asking.
Frankie have you tried the on/off x 3 or 4 times of the ign key BEFORE trying to fire up?
Could be a failed non return valve.


K - I have swapped the FPRV with a known good spare - no difference. The FPRV/regulator is rated at 3 bar (about 45 PSI) which is what I am now getting in the fuel rail.

Robin - Yes, the pump runs to “prime” the lines and then cuts out as it should (to prevent flooding) but no difference. AFAIK the non return valve is in the pump (do correct me if I am wrong) and therefore replacing the pump should have replaced the valve. Anyway, I get 45 PSI on ignition and that pressure holds up when ignition is turned “off” - so I have discounted the valve.


(Robin O'Connor) #5

Ok so fuel pressure looks to be ok, when were the spark plugs last replaced?

(Veekay) #6

When was the fuel filter changed last?

(Maynard) #7

Is there an anti-back flow valve? On the older models it was attached to the fuel pump, on the new models, I don’t know.


I changed the fuel filter and plugs within the last 5,000 miles.

With a good pressure and adequate flow (which I measured at 500ml in 10 seconds) I am beginning to suspect a sensor/electronic fault. I need to start eliminating possibilities - I have a full on day today but if I have time when I get back I will try injecting a little fuel into the throttle body before starting to see if that additional enrichment helps and go from there.

The issue does seem to be cranking related - once it fires and runs its fine. I’m racking my brain as to what is different about cranking? I read somewhere that the ECU fires the injectors 3 times per revolution on cranking and over 200rpm reverts to one per revln.

As further background I have also cleaned the throttle body, idle air valve, checked for induction leaks, put a meter on the TPS, changed the CTS, changed the CPS and more.


(Jimandhelen) #9

CPS? A dodgy CPS can run but be hard to start.




Thanks Jim - I’ll swap it over with another and see. If that is the cause I will be a bit miffed - I replaced the CPS only last June!


(Veekay) #11

Check ECU for corrosion.


CPS - substitution with another sensor makes no difference - R=1,250 ohms
Injecting a few ml of fuel into the throttle body - no difference
MAF meter signal gives 1.44v at idle and 2.22v at about 2,000 rpm - seems OK
TPS gives 0.85v closed and 4.8v fully open - seems OK
CTS gives reasonable readings
No corrosion on ECU terminals.

How can I test the stepper motor/ISCV? I hear it travelling in and out on ignition on/off but can’t tell if its cranking position is correct?

I haven’t checked the O2 sensors - but they were replaced only last year.

Thanks for the advice so far.


(Robin O'Connor) #13

TPS sounds high at base setting it should be lower than what you have, somewhere around .4-5v?


My mistake - reading form the wrong line.

My electrical spec gives the range as 0.58 closed to 4.8 fully open.

My actual readings were 0.56 closed to 4.7 fully open.


(Veekay) #15

A manual reset of the IACV seems to just be a test to see if it is working, but give it a go:

  1. Run engine until you reach normal operating temperature.

  2. Switch ignition off.

  3. Switch ignition on, wait 5 seconds, then disconnect the connector to the idle speed controller.

  4. Switch ignition off.

  5. Wait 15 seconds then reconnect idle speed controller.

  6. Repeat two more times.

  7. On last repeat, do not reconnect idle speed controller.

  8. Start the engine.

Check the base idle speed, which should be at 550-600rpm. If it is, I believe the IACV is working. Once you shut the car off, it will resume behaving badly because something else is at fault.

Switch ignition off and reconnect the idle speed controller. Start engine and check to ensure that the idle speed is between 650-800rpm…it probably won’t be, but if you were successful in the previous step, I think the IACV can be ruled out.


OK. So I did all this. On the last re-start (item 8 with controller disconnected) the engine fired and ran up to about 700rpm but as soon as the starter was released it stalled out. Three times. On re-connecting the controller after this it started (very long cranking as usual) but eventually fired up and idled correctly at 750 -775. What’s the next step Veekay?



AFAIK base idle is adjustable on the AJ6 throttle body but not on the AJ16 throttle body, so I am unsure as to the significance of the manual reset/test suggested above and the result.



Checked out a few more possibilities today.

All coil pack resistances are the same but the values are 1.2 ohms; the workshop manual says 0.75 ohms. Any views?

Plugs are all very sooty but gaps correct. Possible over fuelling with new pump, (was very lean and plugs were white with the old pump).

All injector coils are at 16.3 ohms - haven’t had the injectors out to check spray pattern yet.

Cranked engine for a long time and wouldn’t fire, so took a plug out - bone dry and no smell of fuel. I know there is good pressure in the fuel rail so am thinking now that injectors aren’t firing on cranking for some reason.

May get the injectors out and check them tomorrow if I have time.



I tested the O2 sensors today and found one had gone bad - didn’t suspect that as I replaced both a year ago last week to get through the emissions test. Changed the faulty sensor and now readings all good - hope I will have cured the sooty plug issue…BUT… the long cranking problem persists.


(Veekay) #20

Wait, starter released??? You mean by key?

I’m not sure what you’re saying here.

If after #8 you were able to fire up the car and it only got to 700rpm, in park, with the engine warm, I’m beginning to suspect the IACV. The test should have idle at or below 600rpm. With the harness disconnected it isn’t listening to the ECU or anything else. If it’s not at the ideal 580rpm idle, then it’s not doing it’s job well.