Fuel pressure is good, up to 40 psi. Injectors good, pressure drop with pulse tester and noids fire. Good spark at plugs, new distributor pickup module. Timing has not changed since it ran. Acts like it is just about to start, then doesn’t and goes back to just turning over. Any ideas as to what is wrong?
What does ‘pressure drop with pulse tester’ mean, Kim?
Rail fuel pressure should be around 36 psi, but something else is likely wrong - did you check if the plugs were wet after cranking? Did you set the the pick-up gap correctly when replacing the pick-up?
A crude test; remove plugs and dram each cylinder with a teaspoon of fuel, replace plugs and crank. The engine should fire up and run for a couple of seconds - if it does, it is a fueling problem. If it doesn’t; it is an ignition problem. It’s important to verify which is the issue…
I assume this an xj6 with the CE ignition, and not an xj40 which belongs to a different forum…
Frank
xj6 85 Sov Europe (UK/NZ)
Smell the exhaust after cranking and if you don’t smell tons of fuel, turn the ignition on, tap the bullet connector along the water rail about ten or 20 times, try to start the engine. Does it crank evenly indicating compression in all cylinders?
Thanks for responding. This is not an XJ40. It is a 1987 XJ6 VDP 4.2L.
I have an injector testing unit that sends pulses to the injectors (see pic or link).
[Amazon.com]
With the key on, I hold the AFM open to raise the fuel pressure to 40 psi which drops to 36 psi when released. With several pulses from the tester mode 2, it drops about 4 psi on each injector.
Mode 1: the tester will output 1 pulse, whose pulse width is about 250ms.
Mode 2: the tester will output 50 pulses, every pulse’s pulse width is about 7ms.
Mode 3: the tester will output 100 pulses, every pulse’s pulse width is about 3.5ms.
Mode 4: the tester will output continuously at the rate of 50 pulses per about 1450ms, every pulse’s pulse width is about 7ms.
Pickup gap set to .014" as recommended. Spark looks good at plugs when out and is clearly visible. Plug gap is .035", platinums. I know that compression pressure makes sparking more challenging for the ignition system but I can’t check that.
I did put some fuel in the plug holes and tried that. Didn’t seem to be definitive.
I unplugged all of the injectors and cranked it, thinking that it may be flooding. After about 10 seconds it seemed to start firing and then went back to just cranking.
When cranking with the plugs out, it spins freely, with plugs in it labors like it should with compression. I will do a compression test cold just to check that. It seems to crank evenly, plugs in.
I thought my cold start injector was bad. I took it out and test fired it with the tester under pressure. I didn’t get any discharge. I purchased another one which doesn’t click or spray either. This shouldn’t prevent starting with putting gas in the plug holes though, right?
Is the bullet connector the trigger to the ECM? A noid tried on each injector plug does light up when cranking.
I did smell fuel during the cranking process at one point. See my reply to Frank for other details. Thank you for responding.
Yes, the ECU fires the injectors every third pulse aka once per revolution.
I‘ll think about the rest but seems like you have enough spark and compression that it should be able to start. If more fuel doesn’t help recheck timing (loose distributor clamp?) and check plugs after cranking.
[quote=“Kim, post:4, topic:426006, full:true”]
I have an injector testing unit that sends pulses to the injectors (see pic or link).
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The purpose of the kit is just to open the injectors - either for cleaning them or to measure amount of fuel delivery. It’s fine for the purposes - but in principle the injectors must be out for checking or cleaning. Verifying that the injectors are actually spraying - and deliver the spec amount of fuel…
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With the key on, I hold the AFM open to raise the fuel pressure to 40 psi which drops to 36 psi when released. With several pulses from the tester mode 2, it drops about 4 psi on each injector.
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Assuming the fuel pump is working all time during the testing; the fuel pressures should be around 36 psi. Fuel pressure is unaffected by AFM flap, ign key position or or fuel used.
The ECU does no ‘know’ fuel pressure and has no influence on it. The fuel pressure regulator is there to maintain constant fuel pressure, but is also connected to manifold vacuum. Varying rail pressure to ensure a constant pressure difference between rail and manifold. This is essential to ensure that the ECU delivers the correct amount of fuel…
So your fuel pressure regulator is ‘abnormal’ and strongly suspected faulty. However, whether this is the cause of your problems I cannot state - it implies 10% more fuel delivered than spec…
The trigger signal goes from coil ‘-’ via the bullet connector to the ECU - which ground the injectors for the duration of the pulse. Which varies with load, but is around the 50 ms mark…
Have you actually checked the coil connections - though with test lamp flickering during cranking, the ECU is ‘doing something’…
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Plug gap is .035", platinums. I know that compression pressure makes sparking more challenging for the ignition system but I can’t check that.
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You can triple gap a spare plug to compensate for that, and connect it to any plug lead - to constantly monitor sparking. This will not interfere with starting - the engine will start and run on 5 cylinders if everything else is working…
If the engine did not immediately fire up with ‘dramming’ the cylinders; it implies an ignition issue - or overfueling drowning the spark…
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I will do a compression test cold just to check that.
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That’s highly advisable; it verifies engine itself being in working condition - though it must be very bad to prevent starting…
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I purchased another one which doesn’t click or spray either. This shouldn’t prevent starting with putting gas in the plug holes though, right?
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Right; the intent of the CSI is just to briefly add fuel during cold starting. Duration vary with engine temp, but if coolant temp is above 35C the CSI will not spray. Also, once triggered, the CSI will not spray again on a repeat start - the CSI needs some time to reset, to avoid overfueling…
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Frank
xj6 85 Sov Europe (UK/NZ)
His symptoms could point to flooding/ overfueling. Maybe the mixture is indeed too rich for starting (it wants to fire after unplugging the injectors and it doesn’t want to fire with extra fuel).
If the pressure regulator is indeed reading high that may be a good place to start.
Possibly too much fuel. Possibly from a faulty coolant temperature sensor. CTS. To quick test, remove the harness connector from the sensor. Use a short jumper wire plugged into the harness connector slots and try another start. This effectively removes the sensor from sending a voltage signal to the ECU. The ECU will be “tricked” into thinking the engine is at operating temperature and not add any additional fuel via an increase in injector pulse width. CTS failure is a common problem. Note…this will not test the connector or the wiring, either or both which may be faulty. Another story for later. SD Faircloth
Yes, Kim…
But after cranking and non-start; check plug(s) - if wet engine indeed gets too much fuel…
Frank
xj6 85 Sov Europe (UK/NZ)
Well wonder of wonders. I went to check the plugs after sitting and watched one out to see the spark. It acted like it was going to start so I put the plug back in and it did with some difficulty. Lots of fumes and rough at first but it ran to temp. I was cranking it to see if the plugs got wet as recommended but no need. Thank you all for helping. I very much appreciate the support. The fuel pressure is 30 psi running going to 40 psi with the regulator unplugged.
I can totally relate to Sisyphus. I wonder if he was the original Jag owner.
With the vacuum hose detached, you should have 36 psi, Kim - and idling with hose connected; some 28 due to manifold vacuum.
So you have either too high fuel pressure, faulty regulator - or more likely, an iffy gauge. Are your staring problems gone? Sisyphus did the same thing over and over - with the same result each time…
Frank
xj6 85 Sov Europe (UK/NZ)
Replaced collapsed left fuel tank. All flow and newer solenoids operating as they should. Had a low idle stalling problem which I fixed with a new gasket on the cold start injector. Problem now is roughness in running. How much of that could be due to the injectors needing to be cleaned? Fuel pressure is 34psi normal and 42-44psi with regulator unplugged.
You need to verify your gauge, Kim - fuel pressure reads too high. Excessive fuel pressure may cause rough running - but arguably, it may be just a misreading gauge…
Fuel pressure is tested without the engine running - it should be 36 psi +/- a tad as spec. Though disconnecting the regulator’s vacuum with the engine running should read the spec pressure…
Is the engine running rough at all rpms/loads - or just in idle? Have you cheked compression…?
Frank
xj6 85 Sov Europe (UK/NZ)
I used a digital tire pressure gauge to compare against the fuel pressure gauge and they were within 1 lb. at 36psi. I hooked up a hand vacuum pump to the fuel regulator and sucked it down to where the fuel pressure was 32psi, then clamped it off to hold. It seemed to idle and run better but the plugs are black, not wet. Could the AFM be off?
Put it this way, Kim; if the pressure gauge is correct, your fuel pressure is too high…
…which means the engine is running fat, which means black plugs - and may cause erratic running. As long as the plugs are firing they will not be wet - the xk engine is tolerant to fat running, but there is a limit…
It’s essential that fuel pressure is correct - nominally, with engine running, the fuel pressure would be around 28 psi.
Certainly, fat running may be caused by the AFM, among other things, but it is pointless to search for other faults if fuel pressure is incorrect.
In principle, mixture should be set by exhaust CO content - using a gas analyzer. Sooty plugs does imply that the engine is running too fat - but not how fat…
Frank
xj6 85 Sov Europe (UK/NZ)
What would cause the pressure regulator to run higher than normal, 42+ psi without vacuum?
The pressure regulator is primarily a spring valve, Kim opening to return line at specified, around 36 psi - bleeding off fuel to maintain spec pressure. In addition there is manifold vacuum acting on a membrane that ‘helps’ the spring opening the valve - lowering rail pressure. The idea is to maintain a constant pressure difference between rail and manifold - ensure constant fuel delivery…
That fuel pressure is therefor higher without vacuum (disconnected or engine not running). However; the suspicious bit is too high pressure - which may or may not mean a faulty pressure gauge. Or of course a faulty regulator…
Frank
xj6 85 Sov Europe (UK/NZ)
Is there a case to be made for replacing the fuel pressure regulator that is not leaking, based on too high of a pressure? I used a hand vacuum pump and sucked it down as far as it would go, about 27"hg which gives me 30psi on the fuel pressure. It does run better and the new plugs are not as sooty (after cleaning and trying again). I used a stethoscope and the injectors do not seem to be missing at idle or revved up. I used a spark plug noid and the spark is regular, same rpms. I am replacing the wires anyway. Obviously, the injectors are injecting but I know that dirty ones don’t make a nice spray pattern. Is a poorly spraying injector able to cause missing and not just poor combustion? I have an injector cleaning kit, spray gumout with an electric plug to hold the injector open. If rough running can be caused by a poor spray pattern out of the injector then I will take them out and check/clean them. Forgot to mention, my manifold vacuum is 18"hg and at the regulator hose as well.