1997 Nissan truck rough idle questions

[Luv Nissan]

This is my first post and I appreciate any and all feedback.

Vehicle:
I have a 1997 (I bought new in 1997) Nissan pickup with 197 K miles on it. It has a KA24 engine and a 5 speed manual transmission.
The engine is completely original except for the valve job I had a shop do 2 years ago and the new timing chain / oil pump / water pump I had a shop do 6 months ago.

Symptoms:
Prior to the timing chain replacement the idle was little rough but I had assumed that new plugs and wires would resolve the issue. So I don't believe the timing chain effort had any root cause. Even after some fresh ignition parts the rough idle gradually got worse in daily driving to the point that I had to keep my foot on the gas pedal at stops to prevent the engine from stalling. At idle the engine would cycle between 500 and 800 RPM. While driving at higher engine RPM the engine runs great. There are no engine fault codes showing.

Research thus far:
I've done plenty of online research thus far and this appears to be a fairly common symptom but the root causes seem to vary quite a bit.

Background:
I did a smoke test and found some leakage around the base of the fuel rail where the number one injector sits in the intake manifold. I'm currently waiting for some new fuel rail cushions (I actually had to order those through a dealer) and injector seals to reinstall everything. While I'm guessing the vacuum leak certainly needed fixing it didn't seem so significant that it would been the only root cause. I won't know until I get it back together.

Online research has also pointed toward the IACV, MAB, FPR, and injectors.

Has anyone else on the forum had similar issues with the KA24 engine?
I will also post my results as the project develops.

Thanks

[ezb57e]

It may not have anything to do with it, but there is a ground wire splice , a black wire in the main engine harness near , usually close to the #4 injector, that can be a problem. The original was a brass crimped one, but we soldered it up. It affects the signal from the TPS, ECT, and MAF.
If you can get a data stream and watch the signals from these components you can watch them while moving the harness.
Good luck

[Luv Nissan]

Hey ezb, thanks for that input!

I haven't read anything about this splice before. This gives me one more potential for solving this issue.

My diagnostic approach / equipment set is most definitely low tech / low budget. My vacuum leak / smoke test tools consisted of a roll of masking tape, a length of vacuum hose, a hose barb, and a cheap cigar......low tech but found a leak around the base of the #1 injector.

I just received the new fuel rail cushions and injector o-rings today. I did find one other post in the forum from 2011 indicating a similar symptom from a rotted fuel rail cushion / injector seal leak.

I intend to re-install the fuel rail tomorrow.

I'll post my results.....thanks again

[Q451990]

My diagnostic approach / equipment set is most definitely low tech / low budget. My vacuum leak / smoke test tools consisted of a roll of masking tape, a length of vacuum hose, a hose barb, and a cheap cigar....

That's awesome! If you're looking for a really small leak you can turn your intake into a reverse hookah pipe and invite some friends over! I'm ashamed to say I never thought of it. Although I'll have to step up the price point of the cigar up a bit...



Heath

[Luv Nissan]

Hey Heath,

I wish I could take credit for the idea but someone got there way ahead of me.

I went through the usual carb sprays routine (engine running) and didn't find the leak. There were no audible clues either but still I felt the symptoms were at least partially due to a small vacuum leak somewhere. Just pulling a vac with a pump wouldn't have told me where the leak was so I had to work in reverse. Since a smoke test is done with the engine off you can really dial in on a small leak. The commercial machines are in the $800 range. YouTtube has numerous vids where people have rigged aquarium pumps to cigars which was quite ingenious but still seemed like more effort than really needed. A guy named Scotty has a vid using the manual cigar method. The real downside to this method (if you're not a smoker) is getting rid of the taste.

Anyway I found it a cheap and effective way to pinpoint a vacuum leak and I hope others can use the idea too!

Cheers!

[Luv Nissan]

Ok so here's the progress so far.

I ran a smoke test and found a leak coming from under the #1 injector cushion.

I removed the fuel rail and found some rough machining chatter marks in the manifold well where the injector cushion sits.

I cleaned out all 4 wells and replaced the fuel rail cushions as well as the #1 injector o-rings.
I included a thin coat of black RTV on the bottom of the #1 cushion to fill the rough machining marks.

I ran another smoke test to confirm the leak was resolved.

As suspected resolving this leak was not the only issue however it did make a dramatic difference.

Now the engine will idle (after some adjustment of the idle screw) however it does migrate about 150 RPM's after its warmed up. I pulled the plug on the MAF and it made no difference (likely not the cause). I unplugged the IACV and the RPM migration decreased to about half. I unplugged the TPS and the engine tried to die.

Even though the truck is very driveable now I've still got more investigation to do and am focusing on the IACV, the TPS, and possibly the fuel delivery system. After looking over the throttle body configuration I'm probably going to remove the entire throttle body to replace the IACV....there's really not much working room.

That's what I've had time for lately. I'll post some pics If I can get some advice on how to do that.

This is actually my first fuel injected vehicle so it's been quite a learning experience.

A couple of things I've learned:
- high pressure fuel injection hose is $ 6.50 per foot
- every module, regulator, or sensor involved is $ 75 - $ 115.

My advice at this point is look for possible vacuum leaks first....