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Poor Fuel Economy on RB25DE


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Posted (edited)

Hey All,

I have a R34 25GT thats experiencing high fuel consumption. I'm getting around 6-7 km/L on highway, so about 15L / 100 km. I used to get about 600+ km out of a full tank (65L) on highway so something just seems to be off. When I use the consult cable, I noticed my ignition timing has been retarded back to 5 at idle when the car has warmed up. I think it should be 15 or so. The car doesnt have any mods, and running stock ECU. Also, no codes come up when I read the ECU

Some of the stuff I've tried so far:

  • Change O2 Sensor
  • Clean AFM
  • Change Fuel Filter
  • Change coil packs
  • Change spark plugs
  • Check Fuel Pressure (seems fine according to workshop manual)
    • 41 PSI when fuel pump is primed but car is off
    • 38 PSI at idle
    • 46 PSI when vacuum hose pulled
  • Check VTC Solenoid - Clicks when 12v put across it, gave it a clean anyway and put it back in the car, but no change.
  • Check coolant temp sensor - seems to be working via Consult, and temperature guage on dash will rise up to just below middle

A few questions:

  • Can leaking fuel injectors / bad spray pattern affect timing?
  • I'm wondering if I may have skipped a tooth, but really not sure how possible that is.

I've added the logs here from nissan data scan in case someone wants to help take a look.

Really unsure what to do next, any help would be appreciated.

Thanks!

 

 

 

 

 

 

 

20224151332.log

Edited by _scott
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low idle timing indicates inability of the ECU to control the idle speed down with the IACV, so it is reducing timing to get the speed down. This implies a vacuum leak, which shouldn't cause high fuel consumption (should make it run lean, if anything) but maybe the closed loop O2 control is going overboard.

Look for vac leaks with spray can of carby cleaner.

Also ponder if the air filter is too dirty. You made no mention of looking at it. Also, if that's not the cause, ponder if the cat has collapsed.

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Ill take another look at all the vacuum lines. Maybe I missed something along the way.

I did do a new air filter about 10K km ago, but Ill take another look at it. What is the best way to check if the cat is clogged/done ?

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Nvm, think a clogged cat is pretty straight forward. Literally can’t see through it, and collapsed cat is the honeycomb had disintegrated.

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It's possible that your air leak is not an external one but an internal one. Stuff like the cold start valve/intake air regulator as they get old can struggle to close properly and ends up bypassing a lot of air all the time. Your O2 sensor seems to be healthy and oscillating quite a lot. Makes me think I need to replace my O2 sensor if I'm honest, mine seems a bit lazier.

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On 5/5/2022 at 3:05 PM, _scott said:

Ill take another look at all the vacuum lines. Maybe I missed something along the way.

It's not just lines. It's plenum gasket, inlet manifold gasket, TB gaskets, etc.

Whilst Joshua is correct that an internal vacuum leak is also possible, and an internal leak** could in fact result in the low idle ignition timing also, it won't cause the high fuel consumption. Well, it's even less likely to cause it than a normal vacuum leak from outside would, and even that doesn't seem like the normal result of a vac leak.

** What he really means is a dead or dirty/sticky IACV not being able to close enough to bring the idle down.

 

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Or somewhere your intake plenum isn't flat and sucking in additional air, then your closed loop O2 control strategy is adding in more fuel to compensate for the false positive lean condition.

The above could apply to a pinched injector seal

The above above could also apply to an exhaust manifold leak, where additional air is being sucked in through a venturi effect and once again your o2 is reading baloney. 

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Have you used injector cleaner fuel additive? (My Pulsar was having trouble not stalling every time I pulled up at a set of lights. Threw a bottle of Nulon in the fuel, no more stalling.)

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Posted (edited)

@GTSBoy  Makes sense. I'll give the carby cleaner method a try, I'm probably missing a leak somewhere. Any areas I shouldn't spray around? I assume near the spark plugs as it could ignite. I'll also take a look at the IACV and make sure it's clean. I did however clean that out fairly recently.

@Dose Pipe Sutututu I did take a look at exhaust leaks and there doesn't seem to be one. I sprayed soapy-water up near the exhaust manifold but didnt see any sign of leakage.

 

My valve cover gaskets are leaking, could that possibly cause a vacuum leak or nah?

 

Edited by _scott
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On 5/5/2022 at 9:22 AM, _scott said:

@GTSBoy  Makes sense. I'll give the carby cleaner method a try, I'm probably missing a leak somewhere. Any areas I shouldn't spray around? I assume near the spark plugs as it could ignite. I'll also take a look at the IACV and make sure it's clean. I did however clean that out fairly recently.

@Dose Pipe Sutututu I did take a look at exhaust leaks and there doesn't seem to be one. I sprayed soapy-water up near the exhaust manifold but didnt see any sign of leakage.

 

My valve cover gaskets are leaking, could that possibly cause a vacuum leak or nah?

 

Valve cover leaks are vacuum leaks in a MAF system. The degree to which that matters is likely pretty minor in practice unless you have a gaping hole that you've been ignoring but it does contribute a little.

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Spent some time spraying carby cleaner around the engine yesterday, but wasn't able to find any leaks. checked around intake valve body, intake manifold, injectors, vacuum lines, breather hoses.

Ill be checking my exhaust manifold for leaks, iacv and carb tomorrow.

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@Dose Pipe Sutututu I dont have a smoke tester. Maybe I should go invest in one...

I tried checking my exhaust mainfold for leaks, seems fine. Took a look at my IACV, pretty clean, so doesnt seem to be an issue. 

I also remove my exhaust and checked the cat, hasn't disintegrated and doesnt seem clogged.

Anybody have any idea what to check next?

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ok, I bought a smoke machine thanks to  recommendations from @Dose Pipe Sutututu and found a leak instantly.

Looks like the gasket between the throttle body and intake is cracked and possibly letting air in/out. 

Since shipping times are slow as hell from Japan, what would be the best way to repair this temporarily?

 

 

boot1.jpg

boot2.jpg

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You could try some permatex ultra grey, or ultra rubber.

Might get you out of a pinch. But there is no saving that long term.

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On 09/05/2022 at 8:28 AM, Trex said:

You could try some permatex ultra grey, or ultra rubber.

Might get you out of a pinch. But there is no saving that long term.

This would be a great temporary, temporary permanent work around until your new gasket comes.

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2x internet points to me! That's a bad looking crack. It could be responsible for your troubles, but it might not be severe enough. Look forward to your report back. This sort of stuff is great, because you have a problem that makes you start looking at the car, and you turn up all sorts of things that you really should fix, and you fix them all trying to solve the problem.

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On 5/8/2022 at 3:16 PM, _scott said:

ok, I bought a smoke machine thanks to  recommendations from @Dose Pipe Sutututu and found a leak instantly.

Looks like the gasket between the throttle body and intake is cracked and possibly letting air in/out. 

Since shipping times are slow as hell from Japan, what would be the best way to repair this temporarily?

 

 

boot1.jpg

boot2.jpg

I order a fair amount of parts from Japan(Amayama) and it takes roughly 2-3 days to arrive in Quebec snail mail. 1 day if you pay extra.  If you're in Australia which is much closer, should take half that. 

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On 5/12/2022 at 8:21 AM, TurboTapin said:

If you're in Australia which is much closer, should take half that. 

As if it's "much closer!!" It's like, 20% closer, if that. 8000km vs 10000km

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