Idle too high-3.5l petrol

[disco stu]

Thanks Stu. That is some good info. Funny thing is that I came across talk of a particular idle reset procedure somewhere that was different to the normal thing, one where you run it for a bit, turn ignition off, back to on, do something else, start car, wave your arm in an anticlockwise direction for 2.5s, turn car on and off 3 times......that sort of thing. I just couldn't find the actual procedure anywere, just reference to it

That's interesting that it resets long term fuel trim every 15min (or only 15min worth of data at a time). I gather most cars store them for a lot long than that. On our camry (1998), if I switch from LPG to petrol the car runs like an absolute dog and burns heaps of fuel. I have to pull battery terminal to reset the ECU and then it runs fine, but it makes it annoying when you run out of LPG on the way to work and the car is chugging and coughing along

[stumagoo]

When I change fuel brands (BP to Shell or visaversa is the biggest change) I see it the most.


I have a wideband O2 sensor with a gauge to monitor my setup and if I have been running Shell 98 and am nearly empty - I put in nearly 80 litres of BP and my O2 sensor is reading rich for bang on 15 minutes it then goes back to normal (and often starts to ping) - if I put Shell in after a tank of BP I read lean for 15 minutes of driving (it has to be 15 minutes continuous as it resets the counter if the engine is turned off) then comes right ----- when I was dataloging for the tune I also noticed this in the short term fuel trims

The irony is when I use Shell after a tank of BP even though the o2 is reading lean I dont ping....

I have seen many reports that BP 98 ultimate in particular allows people to get better mileage from their NL and I suspect that there is an addidtive that is making the o2 read rich when it should not be.... a lean engine will potentially have a sublte increase in power (many state BP98 feels more powerful) and of course if cutting injector pulse width time you will run more economical....

my engine however is on its limit and BP98 for the first 15 minutes is telling my o2 its rich when it is not.... I have seen comments from people who have detonation issues on 98 they cant solve - I now wonder if this is one of the causes

[HarryStottle]

Disco, I would replace the gasket between the intake manifold and the plenum.

This is a common issue and is almost always the culprit for high idle speeds. If you used aftermarket gaskets, they would have supplied a thin paper gasket for that. The original is a reusable rubber backed steel gasket, a very different and much better gasket. They do get hard after a while though and if leaking best to replace with new.

A common issue is people try to overtighten the bolts and strip the thread in the manifold which is a soft alloy and easy to strip. Once that has happened you will not ever get a seal. I notice your first thought with the rocker covers was to tighten. This is not how to stop the leaks, very clean surfaces and new gaskets with correct torque settings should work though.

If there is a stripped thread in the manifold, a helicoil can solve it, but I prefer to get a good replacement manifold from a wrecker when I repair them.

If I was you, I'd head to MM and get a new original gasket and use the recommended torque settings (which are surprisingly light 18Nm or something similar from memory), and your idle speed will be resolved.

[disco stu]

Thanks Harry. I've got to take off the intake to fix the rocker cover leaks, so was going to play with it then. I did use sealant on that gasket based on Stumagoo's comments (permatex aviation non hardening). I guess when I pull it off that I'll find out where its been leaking, if at all.

I'm a little stumped on the rocker cover gaskets, like you mentioned. Tightening won't be much help, unless I just didn't torque up enough (possible, don't have torque wrench low enough, just used hang pressure on socket extensions). I'm thinking using gasket good on them might be the best option

[erad]

"I'm thinking using gasket good on them might be the best option"
I have said this many times. A "mechanic> overhauled my NL 3.5 engine and used silicone (not gasket goo, but the principle applies) EVERYWHERE. Brand new rocker cover gaskets didn't leak but 3 out of the 6 the spark plug tubes had oil in the after 2.6 km only!. Reason was that the extra gasket thickness raised the covers so that the spark plug tube gaskets couldn't seal properly. The gaskets are 'squixhable' enough to allow installation with no extra sealants. I would use grease to hold the gaskets in place, but other than that, nothing else.

[disco stu]

I do recall you saying that, and it's definitely in my mind. A mechanic mate was saying that the rocker cover deforms on them and it seals alright if you flatten it out. I'll suss that when I get them off

I thought grease would get eaten away by the oil over time. One option I thought was the non hardening permatex aviation sealant. It squeezes out well so won't cause what you mentioned. I'll make a call once I get them off

One thing I hate about silicone type sealants there is how much it squeezes out and can get on rockers, and globs can get into oil system etc. I like being more precise than that

[brw0513]

I replaced the rocker cover gaskets on my 6G74 around 12 months ago and so far no dribbles! There are very minor weeps that I have learned to live with.

I didn't use any additional sealant at all. I think that would make future replacements a PITA.

The new gaskets have a tapered edge that you can "work" into place between the inner and outer shells of the cover body.

I did borrow a mates 1/4" drive torque wrench to make sure all bolts were torqued evenly. And tightened them in stages - all to intermediate torque values and from the centre pair outwards. A consistent torque setting all around may be very helpful.

The design truly astounds me though. Take a close look at them when you get the chance. I think the MM engineers nicked off early and hit the Sake before coming up with the naff sealing arrangement.

My rocker covers are painted Ferrari red and look great It's a pity the intake manifold and accessory plate hide much of the 6G74. It is a beautiful engine when you get to see it.

[erad]

I do recall you saying that, and it's definitely in my mind. A mechanic mate was saying that the rocker cover deforms on them and it seals alright if you flatten it out. I'll suss that when I get them off

I thought grease would get eaten away by the oil over time. One option I thought was the non hardening permatex aviation sealant. It squeezes out well so won't cause what you mentioned. I'll make a call once I get them off

One thing I hate about silicone type sealants there is how much it squeezes out and can get on rockers, and globs can get into oil system etc. I like being more precise than that

Stu:
Silicone squeezing out is disaster - small amounts can get into the oilways and it doesn't break down. Grease will break down with the oil - it is after all simply a heavy oil held in suspension by some lithium soap. But is will squeeze out enough to allow normal tightening of the rocker covers and the small amount of grease that you use is not enough to damage anything else.

[stumagoo]

over torqueing is the fastest way to destroy the covers and get a leak - I have heard of people straighteneing them but you would need to be pretty careful and accurate.

new gaskets should not need any sealant in this case for reasons mentioned above....

The over torquing can not be stressed enough - I have a Nissan engine that has rubber bushes under the bolts they are done up that softly - it really is just a nip

[disco stu]

So, uuum, looks like I didn't nip them up quite enough initially I think that combined with compression of the gasket over the time its been on. Some of the bolts were finger tight, when I know did them more than that initially (although not enough, admittedly).

I checked the torque I could apply using the method I did them up and it was around half or less. Ended up rigging up a scale onto the ratchet and converting the figures to the force I would need over the length of the ratchet. The bolts went in a long way before that force was reached, so its looking like this will sort the problem out.

Although overtightening of them previously is a distinct possibility, going by the other amount of way overtightened or even snapped off bolts that were on the old engine......

Thanks for all the advice guys. Thankful that this one seems to be an easy fix, although one I wish I'd done properly initially so I wasn't redoing it

[disco stu]

Idle still high. I've got new throttle body gaskets in the kit I bought for manifold/plenum gasket, so I might see if that is my issue.

[disco stu]

Think I might have found at least part of the issue. The old lpg system cut the oxygen sensor signal and ran it through the lpg computer before sending it to the cars ecu. I've unplugged the lpg computer and disconnected the wiring from everywhere else. This means no oxygen sensor signal is being sent to the ecu. It's right down at the plug for ecu, so it's a bugger of a spot to work carefully with soldering iron. I thought it would be running rougher than it is, but would have to be screwing with idle speed, I'm guessing car is running in safe mode, or whatever is called

[Tonestar]

Think I might have found at least part of the issue. The old lpg system cut the oxygen sensor signal and ran it through the lpg computer before sending it to the cars ecu. I've unplugged the lpg computer and disconnected the wiring from everywhere else. This means no oxygen sensor signal is being sent to the ecu. It's right down at the plug for ecu, so it's a bugger of a spot to work carefully with soldering iron. I thought it would be running rougher than it is, but would have to be screwing with idle speed, I'm guessing car is running in safe mode, or whatever is called

Stu,
Four things to check if you have an idle that goes up and down, first is to make sure all the vacuum lines to the throttle body are actually connected (check the one at the rear as it has a habit of coming loose and falling free), secondly, pull out the STEPPER MOTOR and test conductivity across the two earth contact pins with the other contact pins (there's a few guides on YouTube) using a multimeter (otherwise known as Idle Speed Controller or Idle Air Control Valve and here's a link to one that suits a NJ 3.5l https://www.autobarn.com.au/aftermarket-stepper-motor and also one at Repco https://www.repco.com.au/en/parts-se...tor/p/A9334497) with a large proportion of idle issues related to this one component. Bear in mind that normal idle speed in park or neutral is around 1000rpm with idle speed in drive at 800 rpm. Thirdly and less likely is the throttle position sensor on the throttle body. Finally, do this regardless of the other three. Get yourself some Carb cleaning spray and clean the hell out of your throttle body (following the cleaner's instructions of course!).

[disco stu]

Thanks tonestar. Idle was high only, not fluctuating.

Pretty much everything you mentioned was done before even reinstalling the engine. I ended up throwing in a different isc, readjusted tps etc etc for no change

I ended up adjusting the butterfly further closed which helped a bit, but shouldn't have had to do it that way. After running the engine for ages it has settled down a bit. It's funny, now when I shut the engine off when it's warmed right up, it will idle really low on restarting. Throttle butterfly is sticking closed as well due to the adjustment. It's definitely not dirty in there.

My next step is adjusting butterfly open slightly to stop sticking, readjust tps and then just live with whatever the idle is