P1105 - Rough Idle, High Short-Term Fuel Trims

[The Critic]

Earlier tonight, I had a car that gave me some challenges with diagnostics. Just wanted to share the experience with the group.

Vehicle: 1996 Infiniti I30, 3.0L V6
Mileage: 109k

Background: We had just replaced the spark plugs, intake manifold gaskets, coolant cross-over pipe gaskets, cleaned the EGR (and associated plumbing) and cleaned the IAC.

Issue: Upon starting up the car (after reassembly), we noticed a fairly rough idle. After a short test drive, a P1105 code was set. P1105 is for MAP/BAR Switch Solenoid Valve. This was the only code found.

At this point, we re-checked the connections to the MAP but found no issues. We could've thrown a new assembly at the problem, but we decided to look a bit deeper.

Relevant Scan Tool Data:

At idle:

Short-Term Fuel Trim, Bank 1: 3%
Short-Term Fuel Trim, Bank 2: 25%

At 2500rpm: No change.

At this point, I suspected that the vehicle was running lean for some reason. We re-checked for vacuum leaks and found none. We then decided re-check the connections at the injectors on Bank 2; at that point, we found that the connector for the injector at cylinder #2 was not fully seated!

Cleared codes, test drove the car again. Fuel trims are now normal, car idles normally and no pending/active codes were found again.

Moral of the story: Don't replace parts before doing a proper diagnostic! When we saw the P1105 code, we could have assumed that we needed to replace the assembly...but the issue turned out to be different.

 

[simple_gifts]

The car talk guys once said that far too many mechanics would chalk up a CEL to "something with the computer" forgetting that "the same problems one had on cars without computers occur on cars with computers"; This is often not appreciated.

 

[Colt45ws]

Originally Posted By: The Critic

Moral of the story: Don't replace parts before doing a proper diagnostic! When we saw the P1105 code, we could have assumed that we needed to replace the assembly...but the issue turned out to be different.


Yea, this is how a lot of people diagnose; just throwing parts at whatever the computer says is wrong, when often it might just be a side effect of what is really going on.
Did you read/see any pending codes?

 

[niero]

I've found dealerships are notorious for just replacing parts rather than diagnosing. I've come to appreciate some of the small shops.

 

[ARCOgraphite]

Cyl bal and fuel trim are often poor spray malf inj due to 1 cyl lean condition. I had issue with split 0 ring on a cis L-jetronic - in the late hard to find w/no ODB LII

 

[HerrStig]

Originally Posted By: niero
I've found dealerships are notorious for just replacing parts rather than diagnosing. I've come to appreciate some of the small shops.
NIce markup on parts.