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4.6 GEMS fuelling issues


Aragorn
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So one of the last bits of the puzzle on this new P38 is the engine.

It starts and runs, but smells VERY rich. It also doesnt feel smooth, lifting the revs up of idle feels almost like the engine is shaking. It doesnt sound missfirey in the same way a 4cylinder car does, but i'm not familiar enough with V8's to tell for sure, and the vibration perhaps suggests that it is infact missfiring. Rev it up to maybe 2500-3000rpm and it smooths out.

It came with a Faultmate FCR, which reports no fault codes. I've connected up a ELM327 device and went into live data mode and had a look at fuel trims. I'm not convinced the software was working properly, but it was displaying LTFT at "-97%" (which may just be an erroneous reading as it never changed, just sat at -97% and both banks were the same) and STFT was fluctuating between 0ish and 5ish. Lambda voltages were being reported as fluctuating between 0.2 and 1.2v. The fluctuating effect suggests to me that as far as the ECU is concerned, its working properly... most factory ECU's seem to oscillate around stoich due to the way the sensors work. If the sensors are reporting stoich, yet its running rich, that suggests a faulty lambda sensor. But i dont want to spend £££ replacing them and find its exactly the same, and it seems a bit unlikey two sensors would fail similarly, and that the ECU wouldnt notice!

The live data showed the coolant temperature appeared to be working correctly, rising smoothly from 40c up to operating temp, so i'm fairly happy its not that.

When i had it in for its MOT test the guy didnt test the emissions because it had a hole in the exhaust, but going my smell alone you can tell its not running anywhere near correctly.

I'm a bit stumped where to look now though?

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Sounds like your lambdas are OK to me, and even if they were dead, they don't do much at idle, so wouldn't really affect that.

I hate to say it, but are you sure it isn't water vapour in the exhaust, rather than running rich? Normally lambdas will read lean if there is a misfire.

 

 

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Theres no visible water vapour, it just reeks of petrol. Catted EFI'd engines usually smell of very little once running and warmed up, where as this smells like some 70's banger with a badly setup carb...

Coolant level has dropped, but the radiator appears to be leaking so i was ignoring any possible head gasket issues!

I've removed one spark plug, and its black as coal with a slight lighter area around the tip. Need to remove the rest but all the LPG carp is in the way.

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intake air temp seemed sensible, was in the 20's

Dont think fuel temp was there, nor pressure, i'd have to check again, and the things all disconnected at the moment due to the oil pressure sensor fix.

Wonder if anyone nearby has a nanocom or similar...

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i've checked one :P It was pretty sooted up.

Once i've fixed the oil sensor and put the front of the engine back together i'm going to get them all swapped with new. All the gas carp is in the way of them and i couldnt be arsed fighting around it the other day. No idea why they couldnt have mounted the gas solenoids up above the inlet, rather than hanging over the exhaust manifolds :(

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Just to update this, got the plugs swapped out this morning for some nice new BKR6E's. Discovered a few things. First of all, no.7 wasnt even plugged in, the HT lead was lying on top of the manifold... I think i mustve pulled it out when trying to access the crank sensor wiring.

All the plugs were properly black/sooty, but no6 and no8 plugs were both black and shiney, whihc i believe generally indicates oil. they were both heavily fouled so i cant imagine were working very well.

Stuck the new plugs in and fired it up and seems MUCH better. No lumpyness, revs cleanly, sounds much happier, doesnt reek of fuel, and from the short drive i had round the block seemed to have at least some power. The brakes also work now after a new accumulator and a bleeding session!

Hopefully it'll now be working well enough to go thru the MOT. Once i get some miles on it i'll need to pull number 6 and 8 again and see what their condition is.

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OK So this has grown arms and legs.

 

First of all some background. When i bought the P38 off ebay, it was sold as spares/repair. Apparently the engine would run for about 15mins, then die. The bloke told me the rear main oil seal was leaking and that had contaminated the crank sensor. Sounded fishy but the cutting-out-when-warm i'd seen before with crank sensors, so i figured it probably just needed a new one.

When i got the truck home, i put a new crank sensor in it, but discovered the one i took out really wasnt all that old. Hmmmm, somones been here before. The Bloke also mentioned he'd replaced the fuel pump. Since i'd got the truck, this problem hadnt appeared at all. But with no MOT i couldnt really drive it anywhere, so it had only been shunted around the drive. I had made it to and from the first MOT without issue though.

So, take the P38 in for its MOT re-test today, Drove it up and down the dual carridgeway probably 6 or 7 miles to make sure it was all running fine, get everything nice and hot then dropped it in for its test. Went back to collect it and the boy tells me he couldnt test it as it wouldnt run, kept cutting out. Great.

So i drove it home, just. Last half mile it was acting very strangely, missing and cutting out and not wanting to idle, before finally stopping right outside the house. Best way i can describe it is as though someone had a toggle switch attached to the coil or fuel injectors, and was randomly just turning them off. I managed to coax it onto the drive, and sat there pondering what was going on. Tried ot start it a few times and it would start, rev up a bit then you could hear the engine had simply stopped firing. Adding more throttle would sometimes help it rev up but it would fart about for a few seconds then invariably die.

I sat about for a bit thinking what it could be, the the first thing i did, somewhat randomly, was pop the petrol cap off, to be greeted with a loud and pronounced hiss. Hmm, vacuum in the tank perhaps? I started it, and it fired up perfectly and dropped to a nice idle, revved fine, ran fine, all seemed fine. OK i thaught, somethings wrong with the tank breather, perhaps hes pinched a line when reinstalling the tank. But lets be sure, i'll take it for a drive and see if i can repeat the same fault.

Drove to the petrol station about a mile away, no dramas, removed cap and there was a small hiss. Put a tenner in and drove home. About a quarter mile away from the house this time, it just dies. I coast into a side road and try to restart, but nope it starts revs abit, pops farts and cuts out. If you really mash the throttle you can just about get the revs up to 2000-2500ish, but it kept dropping, then picking up with a short burst, then dropping agian. OK i thaught if its a fuel delivery issue, i'll see if it'll switch over onto gas. Started it, revved it a bit and hit the gas button. Once gas came on it suddenly sounded much nicer, revved up nicely, but it was short lived, as a few moments later it starts missing on gas as well before stalling. Basically, doing exactly the same thing. So my petrol tank vacuum idea seemed out.

Scratching my head and wondering how i'm going to get it home, i sat about a bit, tried it a few times, then after some pondering, unplugged the MAF... It fired right up. Waited a moment to see if it was shortlived, but nope, kept ticking over. But hmm! its done exactly this before with the petrol cap.... So set off home and it drove home just fine. Much like the trip to the petrol station, it drove with no issues and all worked perfectly.

Back at the house i ran it on the drive for 10 minutes with no issues.

Sooo, whats going on? MAF failure? Fuel tank cap hissing just a red herring?

I pulled the maf off and gave it a clean (not that that ever works). Refitted it this morning and started it up, and sure enough starts perfectly and runs fine. Perhaps the MAF is dying when it heats up?

Lack of an MOT makes it quite hard to diagnose, as really what i need to do is take it for a 15minute drive with the MAF disconnected and see what happens. Going to pop it down to the MOT centre this morning and see if it will run long enough to get thru the emissions test and get a ticket.

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New MAF went on and i took it back for an MOT. Failed again, bloke said it was missfiring and choking out his tester.

Decided to inspect the plugs to see if they shed any light:

https://www.dropbox.com/s/azls39kcog9xjx4/2016-09-27%2017.45.00.jpg?dl=0

These plugs were new, probably less than 10 miles ago. Albeit most of that distance done with what would seem to have been a fualty MAF sensor.

The odd bit, is they're all quite different.

Starting with bank1 they all look "okish" with the caveat that 5 and 7 both have soot covering half of their plug ceramic. Bank 2 is where it gets all weird. Cylinder 2, is the same as bank 1. Cylinder 3 is dark and sooty. Cylinder 5 is darker and sootier, plus also looks oily. Cylinder 7 is VERY sooty and oily with weird deposits on it (i presume burnt oil).

Thoughts? The oil is clearly a mechanical issue, but could be valve stem seals or rings. For the differences in fuelling between the banks I'm thinking perhaps faulty injectors?

I guess i need to compression test it!

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I've been looking into miss fire issues for gems 4.6 because a potential donor has a miss fire. Any history for the coil pack? They appear to be a weak point for older engines. Don't think they're cheap though!

This obviously has nothing to do with the oil issue!

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Spent some time on it today. Swapped the injectors out for the spare Audi ones i had, based on some info from the TVR fraternity.

Ran a compression test. Unfortunately the battery wasnt great (it had gone flat -yey p38's- and had only been charged for about 2-3hrs on a weedy charger) However cylinders 8 and 6 certainly seem questionable, the same two cylinders with the worst plugs...:

8: 11 bar
6: 8.5 bar
4: 13 bar
2: 13 bar
1: 13 bar
3: 12 bar
5: 11.5 bar
7: 11 bar

The test was performed in that order. Unfortunately the battery was about done towards the end, which i think explains the last three cylinders. I suspect had it been fully charged they'd also have kicked out ~13bar.

I'm going to have to re-do the test with a fully charged battery and will try adding some oil to 8 and 6 and see what happens.

Best case i guess its a head gasket failure between those cylinders? Worst case, bottom end is fecked.

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14 hours ago, WesBrooks said:

I've been looking into miss fire issues for gems 4.6 because a potential donor has a miss fire. Any history for the coil pack? They appear to be a weak point for older engines. Don't think they're cheap though!

This obviously has nothing to do with the oil issue!

The coil pack oddly enough looks like someones set fire to it. The plastics all melted on one side. However the most-melted coils happen to line up with cylinders whos spark plugs look good. So while i probably want to replace it, it does seem to be working fine.

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Compression testing was frustrating for me! The tester didn't want to fit in the hole well and did all the tests before remembering test should be done wide open throttle!

Running the tests again with all plugs out once as before with the engine warm and a second time with a teaspoon of oil in the bores should help identify rings or valve/gasket issues. If the readings go up with the oil you may be able to get away with a new gasket or reseating the valves. If you lift the head if those two bores are much cleaner than the others that is supposed to be a sign of water issues.

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I had the inlet manifold removed while testing, so no throttle issues.

Surely if the readings DONT increase with oil in the bores, it suggests its a top end issue, ie valves or head gasket? If they do increase, it points to ring seal?

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