Newbie with a miss fire problem on boost.

[steve0280]

Evening all,

I've had a little search and couldn't find anything specific to my problem.

Had my s14a since March and love it, I've got it on coilovers with a 3 inch cat back with decat and a blitz air filter.

I did the free boost upgrade about a month ago all was well for a few days then I started getting a hesitation/miss fire on boost at 15psi in 4th I replaced the pipe with the original restricted one and all was well again. Last week I put an r spec manual boost controller on and set it to around 9psi in 1st and second then it was pulling 12 psi in 3rd and forth. Went for a test drive and everything was fine then replaced the plugs with some iridium plugs my mate had then it started misfiring and hesitating on boost but it held boost steady on the gauge. Put the old plugs back in but it's still doing it and I'm pretty lost, my mate has said the standard inter coolers are crap and can cause a boost leak could this be my problem. Any help would be much appreciated.

Thanks

Steve

[Jon]

my mate has said the standard inter coolers are crap and can cause a boost leak

There can't be many original S14 intercoolers that are still airtight nowadays Very common problem.
It may seem a coincidence that this has happened in among spark plug changes, but increasing the boost levels will have exaggerated a leak that was already there methinks.

ALso, I'm thinking 15psi is slightly too high for your mods. Do you have the original fuel pump still fitted ? because that could be the next problem. About £70 for a 255 litre per hour genuine Walbro pump should prevent any future troubles there It's another common S14 weakness.

[Chriscooke]

Could be a failed coilpack.

[steve0280]

Thanks for the replies.

I had been told that the intercooler has probably been leaking for some time due to it being a common fault an I guess turning up the boost has made the problem more noticeable. I've been reading up on the fuel pumps too, could it be a fuel cut due to not having an updated fuel pump?

I did wonder about a coil pack but the car drives ok off boost so surely if it was the coil pack it would misfire all the time etc.

[Jon]

I had been told that the intercooler has probably been leaking for some time due to it being a common fault an I guess turning up the boost has made the problem more noticeable.

The only way to be sure is to pressure test all the pipework between turbo and throttle butterfly. There are various different ways of doing this, but basically seal off each end, and introduce pressure somehow. A bicycle pump is too weak to help, and the airline at the garage is a bit OTT. If you can hear, see, or via washing up liquid see bubbles anywhere, there is a leak.

I've been reading up on the fuel pumps too, could it be a fuel cut due to not having an updated fuel pump?

Fuel cut will have you headbutting the windscreen. It is violent, and no-one would describe it as a misfire.
The problem with original fuel pumps, is that they do wear out and fail (100,000miles is a random figure to quote) and can fail suddenly leaving you stranded.
Worse than that, is that before failure, they could be letting the engine run lean. That is why most SXOC people would recommend replacing the standard pump even if there are no bad symptoms today.

I did wonder about a coil pack but the car drives ok off boost so surely if it was the coil pack it would misfire all the time etc.

Weak coil packs will first show their weakness when under heavy load and high revs.

[steve0280]

The only way to be sure is to pressure test all the pipework between turbo and throttle butterfly. There are various different ways of doing this, but basically seal off each end, and introduce pressure somehow. A bicycle pump is too weak to help, and the airline at the garage is a bit OTT. If you can hear, see, or via washing up liquid see bubbles anywhere, there is a leak.




Fuel cut will have you headbutting the windscreen. It is violent, and no-one would describe it as a misfire.
The problem with original fuel pumps, is that they do wear out and fail (100,000miles is a random figure to quote) and can fail suddenly leaving you stranded.
Worse than that, is that before failure, they could be letting the engine run lean. That is why most SXOC people would recommend replacing the standard pump even if there are no bad symptoms today.



Weak coil packs will first show their weakness when under heavy load and high revs.

Definatly worth pressure testing too see if any leaks can be found.


It's definatly not a fuel cut then lol it's no where near a violent as that, the car has only done 60k ( uk car ) so may not be fuel pump bet defiantly will be upgrading to a 225 pump as I want to go stage 1a next year


The problem starts to occur at 3.5 to 4k revs so would this be classed as heavy load

[redsx94!]

Try gapping your spark plugs to 0.7mm

[LED sandwich]

It try a pressure test and of that has no joy swap your coil packs for a known good set then work out which if any have failed from there.

Sent from my HTC Desire HD A9191 using Tapatalk 2

[pippin89]

Firstly a weak fuel pump would not cause fuel cut. Fuel cut occurs when the boost reaches a certain pressure. The ECU recognises that its reached this pressure level by the amount of air flowing through the AFM and therefore if you have a boost leak it will cause this fuel cut to occur at lower pressure. But as Jon has pointed out fuel cut is a very violent hesitation so it is unlikely to be that from your description. Also as someone else said 15psi is a bit high on a standard map and will probably cause you to be running slightly lean. Under 1 bar (14.5psi) is the general consensus on safe limit.
Lesson over, now to the problem
It does sound like a boost leak. And if it is on a standard Wing mount then its almost definite. But do a boost leak check to be sure. If that doesn't show anything then next port of call would be properly gapped spark plugs. Preferably 1 grade colder than standard if you are planning on going 1a. See where that gets you and then report back.
Oh and yeah make sure you swap your fuel pump. If its not weak now it will be soon.

[steve0280]

Small update on this chaps, I pulled out the standard intercooler last night, found a few bits that looked suspect to leak so I took it into work today and got the workshop to run an air line through it whilst straying the intercooler with water and fairy liquid and ended up finding multiple leaks some of which were pretty bad so hopefully I've found my problem. Got a FMIC on order so will report back once it's fitted to see if it's sorted my problem.

[Jon]

I took it into work today and got the workshop to run an air line through it whilst straying the intercooler with water and fairy liquid and ended up finding multiple leaks some of which were pretty bad

I started reading this , thinking problem solved.

Then remembered that my old workshop airlines fed about 100psi, which means fairy liquid will probably show up a few gaping holes in any intercooler about one tenth of a second later.

I hope your workshop chaps were suitably gentle with it to maybe 20psi maximum ,

It probably doesn't matter now

Front mount

[steve0280]

Another small update on this problem, got my FMIC fitted went for a test drive and it's still the same, so checked the MAF and lamba sensor both are ok.
Tested the coil packs today and all hold the same ohms constantly so guess they are ok.

I fitted a 300zx fuel filter today and put a couple of tins of tech 2000 fuel additive I got from work and went for a bit of a drive, was still hesitant coming on boost but after a while it got better and started to boost fine then I'd get the odd splutter coming on boost but it was generally a huge difference, was running a steady 10psi.

So if the problem peraists I still have a few bits to check, was told ignition timing could be slightly out causing my problem?

[ryry92]

Even though you said it seems fine.. try unplugging the lambda and going for a drive. See if it makes a difference

[Jonny Wilkinson]

Its been said twice and you haven't confirmed you've done it yet.

Spark plug gap.

It causes hesitation when coming onto full boost when the gap is set as specified in the manual.

You need to fit NGK copper plugs and close the gaps up to 0.7-0.8 mm

Its free to try if you already have NGK coppers (less than a tenner a set if you havent) and my success rate doing this on cars with similar symptoms is 3 out of 3.

[redsx94!]

^^^^^ do it!!

[steve0280]

Big fat update, gapped the plugs correctly and boom, no more hesitation.

Thanks to all the suggested it, wish I had done it sooner.

[redsx94!]

Your welcome