Factory dual fuel AU getting bad economy on both fuels and won't rev hard

[Bushbasher]

The short version -

- Fuel economy bad on both petrol and gas
- engine won't rev past 4k RPM on petrol
- at 4k RPM it runs clean at 1/2 throttle but grumbles and labours at full throttle
- engine won't rev past 5k RPM on gas but runs clean all the way at WOT although wheezy
- all ignition electrics good
- both fuel systems appear to be good
- had a gas tune
- had injector cleaner
- replaced fuel filter
- replaced crank angle sensor
- cleaned TPS
- cleaned TB
- no vacuum leaks
- exhaust from cat back is less than 3 yrs old and still looks as new, no rust, tarnish or dents
- when the gas tune was done it was plugged in to a diagnostic unit and didn't throw any codes.

any ideas why it won't go and is getting crap economy?

** replacement cat will be picked up tomorrow and fitted.



The long version -

This has been ongoing for over 12 months now to varying degrees. I think I've sorted it out and it comes good for a tank or 2 then starts getting worse again. The failure to rev hard is a relatively new symptom and I'm not sure if it is coincidental, symptomatic or something I've now caused.
Ok, I'll start from the beginning of this latest attempt to cure the bad economy issue. The wagon runs on LPG 99% of the time so I originally thought it was a gas only problem but it can't be, more on that later. ok, the wagon was getting very sluggish, especially at higher revs above 3500rpm and was just running out of stem completely by 4k and the economy was going away plus going up even a shallow rise it was starting to struggle and was downshifting like it was leaning out or something so I got a gas tune done. This didn't seem to make much difference. Then I got a whiff of gas one day so thought it was leaking gas which explained the bad economy and possibly the poor performance if it was leaning out so I traced the leak and fixed it. No change. Then I decided it might just be that the old donk was getting tired so I did a dry compression test on the cold engine. The results were disappointing with 2 cylinders at 90psi and the others at around the 115-120psi range. So then I did a wet test to see if it was the rings or the valves and squirted some oil down each pot and did the test again and all the pressures came up to around the 150psi mark, so rings not valves, bugga! I've subsequently done a compression test on a fully warmed up engine and all compressions came in between 170-180psi so definitely not a compression issue. This is where I discovered it wasn't a gas only problem as when I started the car back up and took it for a drive to clear out all the oil in the cylinders I decided to switch it to petrol to help wash the cylinders down to help clear the oil and discovered the engine wouldn't rev past 4k. Then I realised the engine was struggling to get to 5k rpm on gas. Thinking I had 2 separate problems now I thought I'd fix the gas problem first so just thinking it was the plugs fouled with the oil I put in to the cylinders I changed them for no change. Then I thought it was running a bit lean at high RPM so I gave it a full turn on the high speed adjuster to see if it would help but no joy. So I was stumped. Plugs are good, leads and coil are good, everything runs ok at low RPM just bad economy 420km from 68L highway when it should be closer to 520km. So I turned to the petrol side and I firstly tossed a bottle of injector cleaner in the tank and replaced the fuel filter as it's never been done in 350k km but it made no difference to the problem, still won't rev. So I thought I'd do an economy test just to see and only managed a paltry 600km highway from a full tank. So now I'm thinking it has to be a problem common to both fuels so first I thought it might be a faulty crank angle sensor not letting the timing advance under load so it's been changed, for no joy. I'm confused as to how it can pull to 5k RPM on gas and do it cleanly yet only pull to 4k RPM on petrol and at anything over half throttle the engine grumbles and shudders like the timing has been retarded too much. Put it in gear and floor it and it'll pull to 4k and only run clean to half throttle yet hit the gas button and suddenly it runs clean and continues to rev to just under 5k although slowly and wheezy like it's leaning out. So now I'm picking up a replacement cat tomorrow to see if that makes a difference but If this doesn't fix it I'm running out of ideas.

Anyone got any ideas where to look next? ?



Cheers
Bushbasher

[JC]

I was going to say cat, and I think once you replace it, you will see why I was going to say that (I'm quietly confident it will fix your problem).

[Bushbasher]

I blooody well hope so cos this is frustrating as all get out.

So do you have an explanation why it'll rev on gas but not on petrol?.... because I still can't figure that one out. If it is the cat then why appear to flow more on gas than on petrol?

[JC]

Originally Posted by Bushbasher

I blooody well hope so cos this is frustrating as all get out.

So do you have an explanation why it'll rev on gas but not on petrol?.... because I still can't figure that one out. If it is the cat then why appear to flow more on gas than on petrol?

If the cat fixes it, then the explanation for more revs on gas than petrol is "just because".

The only reason I can think of is that than the 2 fuels are different, and it is possible that gas produces less quantity of exhaust at various revs therefore the restriction takes longer to build. The collapsed cat is a back pressure restriction - some volume of exhaust gas will get through, but once it hits critical volume, it starts forcing exhaust back up the manifold.

My theory is based on my recent experience with our explorer, which has had numerous issues in the 2 months since I've had it mostly comes back to a collapsed cat (2 actually - just one nearly blocked, and one about 50% blocked). Since dropping the Y pipe and unblocking the driver's side cat, we can now get over 3000rpm without the engine light flashing, we can go WOT up hills without the engine coding (WOT at any revs produced a "pull back" effect - like hitting a brick wall in acceleration, and actually caused the flashing engine light to go solid and code). Some of these issues were worse on dedicated SVI LPG (a one off conversion), some not so bad on gas, but I have now converted back to petrol, removing the gas altogether, and until I replace the cats, there are still some issues, but the car behaves better overall on petrol (especially now we have temporarily derestricted one of the cats).

[mik]

I was going to suggest looking at the cat as well, a mate of mine had something similar , over a period of 2 years he had ongoing problems and replaced everything on the engine that could be replaced , and then repeated the process incase something he replaced was faulty, nothing worked although a tiny bit better,
different car similar symptoms, it was an el falcon on gas.
too make a long story short half his problem was the cat,the other half of the problem was a gas installation problem, the earth near the ecu was poorly done , one look under the kick panel showed corrosion where they self taperd the earth wires.
it may be unrelated , but worth a look imo, good luck.

[FGII-XR6]

I agree with the cat. sounds like the cat has collapsed internally and is restricting the exhaust.

[Evgeni]

You can always unscrew the o2 sensor out and see if this makes a difference - just to diagnose before splashing out on a new cat if you think it is collapsed.
The computer will ignore the o2 anyway in closed loop and WOT.

[Bushbasher]

I fitted a new o2 sensor to both cars about 2 months ago and to just make sure it wasn't a dud sensor I switched the sensors between cars 2 weeks ago an it made no difference and the Fairlane is running fine. The only other thing I can think of is the MAP sensor but as of about 3 weeks ago it wasn't throwing any codes.

[Evgeni]

Originally Posted by Bushbasher

I fitted a new o2 sensor to both cars about 2 months ago and to just make sure it wasn't a dud sensor I switched the sensors between cars 2 weeks ago an it made no difference and the Fairlane is running fine. The only other thing I can think of is the MAP sensor but as of about 3 weeks ago it wasn't throwing any codes.

Sorry, should have been clearer - not questioning if it is a faulty o2.
By unscrewing it, it will allow somewhere for the exhaust gases to go if in fact the cat has collapsed. See if there is a difference with the "blockage" removed so to speak.
If it is, then you should ask the question of why it is collapsed and have a look at fuel trims to save your new cat from the same problem.

[Bushbasher]

It's a moot point now as the replacement cat is being picked up by the missus this afternoon when she leaves work and as for why the cat died, (if it is the cat), it might have something to do with it being 350,000km old and having been slammed into the odd rock once or er,.. twice.... or erm, 3.. times... in the last few years. That old wagon has been places it probably wasn't prudent to take it from time to time and has survived.. mostly, although it is in it's 2nd sump, 3rd tail pipe and 4th centre muffler now and only the first muffler was actually rusty and leaking before it was replaced. As for the sump, well, we won't go there as it was the sump, 3 mud flaps and the under tray that day as well as the beginning of the end of one of the mufflers.. For some strange reason my wife just couldn't grasp the importance of me and a mate having to get to THAT particular fishing spot that day...............



Bushbasher

[Evgeni]

Originally Posted by Bushbasher

it might have something to do with it being 350,000km old and having been slammed into the odd rock once or er,.. twice.... or erm, 3.. times... in the last few years.



Bushbasher

That'll do it!

Hope it all works out.

[dieseltrain79]

Probably makes less power on Gas . So less power = less exhaust gasses . Therefore on gas it would require say 5000rpm to produce similar exhaust gas volumes as More powerful petrol gasses @ 4000rpm .

I'm guessing blocked cat should fix it up as well .

Hope that helps exain it .

[Bushbasher]

That makes sense so you might be right. The engine makes 157kw on both fuels but on petrol it makes max power at about 3700rpm or something and LPG max power is at 4650rpm or there abouts.

[Bushbasher]

Well I changed the cat this morning any she now revs out on both fuels. Although, at WOT it insists on up shifting at 4800rpm on both fuels so I've disconnected the battery to reset the ECU as I'm assuming the ECU has tuned itself to the dud cat so going back to default will hopefully sort it out.

Bushbasher

[dieseltrain79]

Good stuff

[JC]

BB -did this end up all good, or still not revving cleanly past 5000?

[Bushbasher]

It's all good JC, if I hold a gear it'll pull all the way to the red line on both fuels but it still insists on up shifting at 4800rpm at WOT if left to its own devices. Having said that, it's roll on acceleration from 100kph on LPG is pretty much line ball with petrol now which I don't ever remember it being before except when it had the Airod variable mixer on it and the high speed adjuster was cranked open. I guess the gas tune it had not long back made a bit of a difference.

Bushbasher

[JC]

Cool, good to know. Stuffed cats are now my specialty, given all the cr@p I've to deal with on the Explorer over the last 10 weeks, and your symptoms matched some of my earlier ones.