MAJOR MISS WHAT SHOULD I CHECK??

[BOUDREAUX]

My 92 q45 has what seems to be a major miss. At idle the vehicle bumbles badly, as I accelerate it gets worse. It does go away once highway speed is achieved. I checked the resistance on the fuel injectors and they all test 12-15 Ω. What should I check next?

Boudreaux

[maxnix]

Fuel pressure after the filter.

[Q451990]

I don't disagree with Brian's recommendation to check the fuel pressure, but that bounce still sounds like a dead cylinder. Remember that it's possible for an injector to be clogged without being electrically defective. I guess it's possible to have a bad coil pack too, but that's a very rare failure.

If you have a laptop I would absolutely invest in one of the USB Consult boards. We're literally a day or two away from being able to perform a power balance test that will pinpoint the exact cylinder you're having problems with...

The cost of the board and software will probably be about the same as having a dealership run the test for you one time.

Heath

[Q45tech]

Why we use BG44k fuel injector cleaner as a diagnostic besides it's regular annual or per oil change usage.

Abnormal for the injectors to deviate more than a few tenths of an ohm from average. depending on harness and ohmeter accuracy hand pressure on probes etc.

Then we pull and examine plugs even if they are only a year old............look for oil leaks into plug chamber.

Knock sensor ohm check 550k+-5%..............ecu coolant temp sensor check?

Wiggle MAF and CAS connector while idling?

Temporarily squeeze return fuel line shut [smooth pliers] for a few seconds to raise idle psi from 34 to 60 psi to blow out trash in injector?

[BOUDREAUX]

What is sensor is the CAS?

What could be causing me a problem with the fuel pressure? I replaced the fuel filter about two months ago.

This problem was sudden.

Thanks,

BOUDREAUX

[96Qowner]

Q45 fuel pumps tend to fail after 100,000. Do you hear a loud hum from the trunk?

CAS is Crank Angle Sensor. The connectors can become iffy and you get weird problems. Pull the connector and clean it up - coat it with some dielectric grease and snap it back together. Do the MAF connector, too (the box on the air intake hose, near the air cleaner).

[BOUDREAUX]

I replaced the fuel punp about 2 years ago. It is quiet as a mouse. I will clean the electrical conections.

Boudreaux

[maxnix]

If that doesn't do it, then check your fuel pressure.

[BOUDREAUX]

The car is under powered by atleast 1/3 at this point. The miss fluctuates from day to day. What is the procedure for checking the coil packs? If one or more packs were bad woult the miss be more consistent? How do you check the fuel pressure? Could the fuel pressure regulator be bad?

Boudreaux

[96Qowner]

Follow-up on Q45tech's suggestions. Until then, everyone is just guessing. Gotta know what your fuel pressure is. If you can get to a Nissan or Infiniti dealer, you can have them plug it into their Consult and get real data - costs somewhere between $100 - 200. Forget the coil packs for now - highly unlikely.

BG44K is cheap - worth a try if you don't want to do the Consult right away.

[Q45tech]

We work on at least a dozen ever week and have for 12 years, in that time we have standardized on a proceedure................most of us can look at a Q towed in or driven in or lay our hands on the hood and be 90% correct in an immediate diagnosis.

[BOUDREAUX]

If I post a pic can you narrow down the possible diagnose!? I am stuggling. I purchased a can of BG44K from Joe at Everything Infiniti.I will let you know if it helps.

Boudreaux

[maxnix]

.....most of us can look at a Q towed in or driven in or lay our hands on the hood and be 90% correct in an immediate diagnosis.

Hands on the hood if you believe!

[silverQ]

My 92 q45 has what seems to be a major miss. At idle the vehicle bumbles badly, as I accelerate it gets worse. It does go away once highway speed is achieved. I checked the resistance on the fuel injectors and they all test 12-15 Ω. What should I check next?

Boudreaux

I use BG chem-tool $3 (from AutoZone w/ Toulene) for deposit problems. Seems like I have this problem also when I use E10 gas, moisture?

[maxnix]

Must use only ISO-HEET with E10. NOT HEET!!!