V6 Misfire Nighmare

[Guest danny.s]

Hi,

Really bad missfire loss of power and sounds like a tank, Its had all the usual causes checked ie: leads,plugs,coil pack,sensors some bits needed replacing but missfire remained the same.

A compression test has been done : 1=175

2=120

3=165

4=122

5=150

6=120

Has anyone got any idea's ??

Thanks

 

 

P Reg 2.8 v6 ghia auto

[dipsomaniac]

have you identified which cylinder(s) are misfiring?

[sepulchrave]

Those compression pressures are suspiciously low on one bank of cylinders, 25% is way out.

 

Head gasket, warped casting, bent valves?

 

Sounds like a strong case for a top end teardown to me.

[Guest danny.s]

have you identified which cylinder(s) are misfiring?

 

Not yet would this be an easy check ??

A different garage has noticed that at least one of the leads is arcing out badly and that the clips holding the large pipe onto to the air filter box have broken (please excuse the lack of technical terms i'm new to all this :) ) This may explain the noise.

 

Those compression pressures are suspiciously low on one bank of cylinders, 25% is way out.

 

Head gasket, warped casting, bent valves?

 

Sounds like a strong case for a top end teardown to me.

 

Damaged valves has been suggested, The garage said it is a massive job engine out strip down

[dipsomaniac]

you need to start by replacing the ht lead(s) arcing. you can buy them separately from ford (

[dipsomaniac]

just looked up the compression tolerances for the v6 AAA engine - 10 - 13 bar. not an expert on pressure but used a calulator to convert to 130 - 170 psi.

[sepulchrave]

Dude, 1 Bar equals 14.3 PSI, get your calculator out again! :D

 

...and danny.s, as far as I'm aware it is NOT necessary to remove the engine to get the head off. Get a new garage, preferably an independant VW specialist.

 

The head CANNOT be skimmed (noticed another poster got ripped off paying for this, didn't feel like telling him) as it is a chamberless heron head and the valve seats are flush with the surface.

[johnb80]

I'd be very tempted to check the valve timing on the bank with the low compressions. If it's out, the valves may be bent BUT setting it right and doing a compression test again is a lot easier than ripping the motor apart.

 

I'm not convinced about head gasket / warped head etc, the pressure are all pretty much the same on that bank.

 

Regards - J

[sepulchrave]

johnb80, how far out do you think the valve timing would have to be for the valves to remain open long enough on the compression stroke to cause a 25% drop in compression pressure?

 

Is it plausible that this could happen without damaging the valve heads?

 

How do you correct the valve timing on an enclosed chain drive without dismantling everything? Why leave the head on when it's the only thing left to remove?

 

I think danny.s needs professional help here, and maintain that an independant VW specialist is the way forward. The only mechanism for the tractor sound he hears is combustion pressure bleeding through valves which are not closing, or leaking from a properly blown head gasket due to head warpage.

 

The only cheap option is to check that the sparkplugs are all torqued down properly, that is the only other way to explain the symptoms.

[Sharanfx]

Leads and plugs are your first point of call as it sound like they need replacing. You would be supprised how bad one cylinder mis-fire is on a V6.

German Swedish and French can sell you a set of leads and plugs at a good price as its a VW engine.

 

Then check your coilpack it could be breaking down to earth. Its a common fault with the VR6 engine.

 

The engine is Chain driven so its highly unlikely the valve timing is out un less your tensioners are compleatly shot, but the chain mould be constantly rattling.

 

Check the Air intake pipe again, make sure its not loose after the Airflow meter (MAF) and sort out the loose clips on the pipe, as your be sucking in un-filterered or possibly un-metered air.

[johnb80]

I have actually had a very similar problem on a 24v Cosworth V6. The idle was dreadful, what appeared like a misfire wasn't, compression was down on one side and that bank was in fact running lean. The inlet cam had jumped 2 teeth. New chains, guides, tensioners and sprockets resulted in a complete restoration of performance and smoothnes.

 

So in answer to your question, YES jumped timing can cause loss of compression (I found it hard to beleive too) and very rough running.

 

Regards - J

[sepulchrave]

...but no bent valves because your V6 engine was an entirely different design, remember the VR6 is actually a dual plane inline engine.

[MrT]

How does the alignment of the cylinders affect a timing chain slip and piston to valve clearance? Is this because there is not separate camshafts per bank?

[sepulchrave]

No there are two camshafts, but they are assymetric since the head layout requires that the ports are in a crossflow configuration, this means there are three long exhaust ports and three short, likewise for induction.

 

The design constraints for this engine were to produce a very compact unit producing reasonable power and fuel efficiency, however combustion chamber design is severely compromised by the layout, hence the heron head and peculiar wedge-crowned pistons.

 

This is not a performance engine by any manner of means, 100 bhp per litre is practically quite difficult to achieve (even a highly modified BL A series engine can approach this level of performance) due to these constraints.

 

Due to these design compromises the engine is forced to run the valve heads very close to the enlarged piston crowns in order to maintain a high enough compression ratio to permit lean burn and emissions performance, consequently any timing slip on these engines tends to affect the leading cam (since they are chained together and driven from a jackshaft which is itself chain driven from the crank) controlling the valves on cylinders 2, 4 and 6 which tend to nip on the piston crown.

 

I'm hoping it's the head gasket since this is much cheaper! However high mileage VR6's have a reputation for camchain tensioner failure which causes these problems. It does sound like only six valves may be affected, often the tensioner (particularly on thrashed boy racer Golfs) fails at high revs causing a catastrophically expensive head wreck. Stop driving it and get it to a specialist danny.s.

 

There, I'm done. Shoot the messenger. :ph34r:

[anadin]

next question ? anyone :ph34r:

[Smilge]

I'm hoping it's the head gasket since this is much cheaper! However high mileage VR6's have a reputation for camchain tensioner failure which causes these problems. It does sound like only six valves may be affected, often the tensioner (particularly on thrashed boy racer Golfs) fails at high revs causing a catastrophically expensive head wreck. Stop driving it and get it to a specialist danny.s.

 

There, I'm done. Shoot the messenger. :ph34r:

 

Nice to here someone who knows what they're on about

[johnb80]

 

Due to these design compromises the engine is forced to run the valve heads very close to the enlarged piston crowns in order to maintain a high enough compression ratio to permit lean burn and emissions performance, consequently any timing slip on these engines tends to affect the leading cam (since they are chained together and driven from a jackshaft which is itself chain driven from the crank) controlling the valves on cylinders 2, 4 and 6 which tend to nip on the piston crown.

 

I'm hoping it's the head gasket since this is much cheaper! However high mileage VR6's have a reputation for camchain tensioner failure which causes these problems. It does sound like only six valves may be affected, often the tensioner (particularly on thrashed boy racer Golfs) fails at high revs causing a catastrophically expensive head wreck. Stop driving it and get it to a specialist danny.s.

 

There, I'm done. Shoot the messenger. :rolleyes:

Would you not agree that IF the timing has slipped the valve opening will be retarded? That being the case, the inlet valves opening later would give greater clearance. The exhaust on the other hand would not :o

 

Regards - J

[Guest danny.s]

have you identified which cylinder(s) are misfiring?

 

Not yet would this be an easy check ??

A different garage has noticed that at least one of the leads is arcing out badly and that the clips holding the large pipe onto to the air filter box have broken (please excuse the lack of technical terms i'm new to all this :rolleyes: ) This may explain the noise.

 

Those compression pressures are suspiciously low on one bank of cylinders, 25% is way out.

 

Head gasket, warped casting, bent valves?

 

Sounds like a strong case for a top end teardown to me.

 

Damaged valves has been suggested, The garage said it is a massive job engine out strip down

[wood73]

if the tensioner has failed, there will be a very noticable rattle, this happened to mine and I ended up replacing the engine as it was easier, i got hold of a second hand unit for

[sepulchrave]

Ahh, and has the coilpack REALLY been tested?

 

I assume the same 'lassoo swinging buggers' are responsible for giving it a clean bill of health.

 

Please stop using crap garages, none of them have a clue what to do with these cars. A main dealer will just throw expensive parts at it on a 'best guess' basis until either you or the problem goes away!

[Guest danny.s]

Ahh, and has the coilpack REALLY been tested?

 

I assume the same 'lassoo swinging buggers' are responsible for giving it a clean bill of health.

 

Please stop using crap garages, none of them have a clue what to do with these cars. A main dealer will just throw expensive parts at it on a 'best guess' basis until either you or the problem goes away!

 

 

Thats a fair comment, Is there a check i can do to test the coil pack ??

[Sharanfx]

Ahh, and has the coilpack REALLY been tested?

 

I assume the same 'lassoo swinging buggers' are responsible for giving it a clean bill of health.

 

Please stop using crap garages, none of them have a clue what to do with these cars. A main dealer will just throw expensive parts at it on a 'best guess' basis until either you or the problem goes away!

 

 

Thats a fair comment, Is there a check i can do to test the coil pack ??

 

The Coil packs breakdown and start to crack and let in moisure. If its not a constantly misfiring cylinder, it cam be hard to locate.

 

Connect a known good lead with a sparkplug on the end touching earth (use a insulated pair of pliers) to earth. Connecting it to each of the outputs in turn a checking for a spark.

 

Its also worth removing the coilpack and inspecting it for dammage or cracks.

 

Its been said before, you can bake it in an oven on low heat to try and dry out any moisture. If it works well after that the coil pack is definatly cracked somewhere and the fault will soon return. If a crack is found you can temp. seal it up with aradite.

 

Has it now had a complete new set of leads? The leads are not rubust, and can easily break if pulled out without a removal tool.

 

You could always get hold of a coilpack to test it out (return it unused if not needed :rolleyes: ).

[Guest danny.s]

Ahh, and has the coilpack REALLY been tested?

 

I assume the same 'lassoo swinging buggers' are responsible for giving it a clean bill of health.

 

Please stop using crap garages, none of them have a clue what to do with these cars. A main dealer will just throw expensive parts at it on a 'best guess' basis until either you or the problem goes away!

 

 

Thats a fair comment, Is there a check i can do to test the coil pack ??

 

The Coil packs breakdown and start to crack and let in moisure. If its not a constantly misfiring cylinder, it cam be hard to locate.

 

Connect a known good lead with a sparkplug on the end touching earth (use a insulated pair of pliers) to earth. Connecting it to each of the outputs in turn a checking for a spark.

 

Its also worth removing the coilpack and inspecting it for dammage or cracks.

 

Its been said before, you can bake it in an oven on low heat to try and dry out any moisture. If it works well after that the coil pack is definatly cracked somewhere and the fault will soon return. If a crack is found you can temp. seal it up with aradite.

 

Has it now had a complete new set of leads? The leads are not rubust, and can easily break if pulled out without a removal tool.

 

You could always get hold of a coilpack to test it out (return it unused if not needed :lol: ).

 

 

Well a few weeks have passed and I have been running the car to and from work, the misfire is still present but up until today it still ran.

The car now refuses to start there is power, lights, radio etc so I

[dipsomaniac]

don't think the clip is relevant. you need to call out a good mobile auto electrician as the price of these parts are too expensive to play guessing games with.

[MrT]

That looks identical to a clip from a photo frame that has a sheet of glass and a backing sheet, but no frame around the edge.