Hi guys, got. 97 p38, 4.6v8 gems, 57k miles, had a full list of parts replaced trying to figure it out, ( plugs, leads, coil pack, inlet manifold gasket, o2 sensors, maf, tps, stepper motor, vac line on back of inlet, oil and filter, air filter, fuel filter, all injectors are clicking, and loom plugs tested with noid lights and ok, cleaned all earth points as well, 110 psi across all 8 cylinders, still runs horrific even after resetting adaptive values, when revving it’s fine, idle settles and after about 10 seconds starts to misfire, local rr specialists are unsure on what could possibly be going on, any of you guys got any ideas other than scrapping the bloody thing?
What's the fuel pressure and does it droop when the misfire starts? 110psi is low on all pots but it still shouldn't cause a misfire, it will just be a bit down on power. About the only thing you haven't changed is the crank position sensor but they normally work perfectly OK until they get hot and then they die completely. Do you have diagnostics that can show live data?
Gilbertd wrote:
What's the fuel pressure and does it droop when the misfire starts? 110psi is low on all pots but it still shouldn't cause a misfire, it will just be a bit down on power. About the only thing you haven't changed is the crank position sensor but they normally work perfectly OK until they get hot and then they die completely. Do you have diagnostics that can show live data?
Gilbertd wrote:
What's the fuel pressure and does it droop when the misfire starts? 110psi is low on all pots but it still shouldn't cause a misfire, it will just be a bit down on power. About the only thing you haven't changed is the crank position sensor but they normally work perfectly OK until they get hot and then they die completely. Do you have diagnostics that can show live data?
Unsure on pressure, will see I I can check this tomorrow, according the some handbook thing I have 110 is about right for that engine, yeah that’s why I haven’t replaced it yet as doesn’t seem faulty to me, and yeah I can access live data, is there something in particular I should look for?
Look at the fuel pressure on the Schrader vlave on the fuel rail, you don't need anything clever, you can use a tyre pressure gauge. With it running, look at the O2 (Lambda) sensor outputs. On a GEMS they are 5-0V sensors with 5V being lean and 0V being rich and should flip-flop between the two limits. If it starts missing the problem is usually it going rich, so you would be looking at the sensor voltages going to 0V. Also look at the output from the MAF sensor and see what that is showing you. If you have replaced with a pattern part, they often give really odd readings.
Gilbertd wrote:
Look at the fuel pressure on the Schrader vlave on the fuel rail, you don't need anything clever, you can use a tyre pressure gauge. With it running, look at the O2 (Lambda) sensor outputs. On a GEMS they are 5-0V sensors with 5V being lean and 0V being rich and should flip-flop between the two limits. If it starts missing the problem is usually it going rich, so you would be looking at the sensor voltages going to 0V. Also look at the output from the MAF sensor and see what that is showing you. If you have replaced with a pattern part, they often give really odd readings.
I’ll try that tomorrow, any idea what pressure I should see? I believe the o2 sensors were both reading 0.14-1.5v at a max, the new mag was just a hunch after being told by someone else to try it but maf is reading 28.2kg/hr, can set stepper motor to 15-30 steps and was okay for a few days till we did a 50ish mile drive and were straight back where it started
Depends what scanner you are using on it. The GEMS is not fully OBD compliant but OBD2 only understands 0-1V Zirconia sensors and not the 5-0V Titania ones we have so a generic scanner will often translate the 5-0V signal into the equivalent 0-1V where 0V is lean and 1V is rich. I've got a Nanocom which shows the readings correctly but I've also got a Launch CReader (V or V1) which translates the 5-0V into 0-1V readings and I think, but can't be certain, that it does show them the correct way round (showing 0V for lean and 1V for rich). Correct MAF readings are 20 (+-3) kg/hr at idle, rising steadily to 61 (again +-3) kg/hr at 2,500 rpm. Fuel pressure should be 34-37 psi but with reference to intake manifold pressure so will vary with throttle depending upon how much intake vacuum there is (higher at idle, lower with throttle open).
Gilbertd wrote:
Depends what scanner you are using on it. The GEMS is not fully OBD compliant but OBD2 only understands 0-1V Zirconia sensors and not the 5-0V Titania ones we have so a generic scanner will often translate the 5-0V signal into the equivalent 0-1V where 0V is lean and 1V is rich. I've got a Nanocom which shows the readings correctly but I've also got a Launch CReader (V or V1) which translates the 5-0V into 0-1V readings and I think, but can't be certain, that it does show them the correct way round (showing 0V for lean and 1V for rich). Correct MAF readings are 20 (+-3) kg/hr at idle, rising steadily to 61 (again +-3) kg/hr at 2,500 rpm. Fuel pressure should be 34-37 psi but with reference to intake manifold pressure so will vary with throttle depending upon how much intake vacuum there is (higher at idle, lower with throttle open).
Ignition on and fuel pressure is between the 34/47 psi, On idle fuel pressure is 29/30 psi (fluctuating) when on throttle it doesn’t move much, may go as low as 28psi
That's low, have you replaced the fuel filter? I probably confused things a bit with my last, higher intake vacuum at idle will result in lower fuel pressure and with the throttle open there's less intake vacuum so the fuel pressure will stay high. If the fuel filter is clogged that could explain why it is OK for a few seconds upon dropping to idle and then start missing. It is likely to be running lean most of the time so after a run the fuel trims will have gone positive to correct it so it is then rich next time it is run.
Did replace the fuel filter this morning again, made no odds to fuel pressure at all, spoke to another local Land Rover specialist and this one seems to think he has the diagnostic computer that’ll work with my p38, definitely seems to be worse when In gear though, it’ll idle in park or neutral around 600rpm, but will remain very lumpy, if you rev it, it’ll drop to around 700 and idle perfectly and on live data, stepper motor will read 17 steps and maf 22.4kg/hr, then after 10/15 seconds it’ll drop even further to the 600 mark and be lumpy, when lumpy, the stepper motor will sit at around 34-45 steps and maf will show 36.2kg/hr, when in gear it doesn’t even stop at the 700 mark, just straight down to lumpy and will stall occasionally but will fire back up like nothing happened,
17 steps is a touch low, optimum is between 15 and 30 but it should be OK. When the revs drop, the stepper opens up to try to increase the revs back to where they should be but for some reason, it isn't doing it. What is a bit weird is the MAF as at the lower revs it should show a lower airflow not higher. When it is idling lumpy, what happens if you very slowly open the throttle? Do the revs climb steadily and the misfire clears or is there a hesitation at the just open position?
See what different diagnostics show and go from there.
If I slowly apply the throttle it’ll misfire and splutter then almost backfire, then it’ll rev up completely normally, the other day I applied the throttle as slow as I possibly could and I got to full throttle before it ‘backfired’ then it revved out completely, but I doesn’t do that all the time, sometimes it’ll rev completely normally and smoothly, then the next time it may do what I just described, or it’ll pick up slowly but continuously misfire until I let off the throttle
It all sounds much like the MAF, all sorts of strange things can happen if they are iffy. Sometimes they won't start at all, other times they will start but not rev unless you keep pumping the throttle, other times they appear to start and run normally but misfire when under load and it sounds like you've got a mix of all options. Genuine ones are still available but at around a grand a time, I doubt many people buy them (which is probably why they are still available) and aftermarket vary wildly, some work OK, some work but give really odd outputs while others just don't work at all. Secondhand original are a better bet.
Worth checking the firing sequence, even for the umpteenth time...
Gilbertd wrote:
It all sounds much like the MAF, all sorts of strange things can happen if they are iffy. Sometimes they won't start at all, other times they will start but not rev unless you keep pumping the throttle, other times they appear to start and run normally but misfire when under load and it sounds like you've got a mix of all options. Genuine ones are still available but at around a grand a time, I doubt many people buy them (which is probably why they are still available) and aftermarket vary wildly, some work OK, some work but give really odd outputs while others just don't work at all. Secondhand original are a better bet.
I do have another second have maf which have all the same readings as the one I currently have, although when I first plugged it in, made it run a lot worse, and would make it literally black smoke like a diesel, even though on live data it have the same readings as the oe one already on it
romanrob wrote:
Worth checking the firing sequence, even for the umpteenth time...