1998 3.5L Spark Plug Wire Troubleshooting

czd3my

New member
I had mine serviced recently and they had an o-ring on the manifold end. I think they seal by being compressed down not sealing around the bore. If I remember right when you tighten the fuel rail down it puts pressure on them, if that makes sense.
 

dieseltechJay

Observer
I had mine serviced recently and they had an o-ring on the manifold end. I think they seal by being compressed down not sealing around the bore. If I remember right when you tighten the fuel rail down it puts pressure on them, if that makes sense.

That is exactly right. They are a harder rubber, and do shrink and crack over the years. You're better off replacing them and the upper o-rings. I rarely trust old rings/seals/gaskets, especially on any vehicle 10-years or older. It's cheap insurance.
 

Salonika

Monterror Pilot
Factory service manual exploded view of rail & injectors doesn't indicate an o-ring on the delivery end, only an "insulator". Is this what you are referring to? I have an o-ring on the rail side of the injector where it seats into the rail, but no o-ring on the delivery end. Anyone have a photo?
 

Howah

Observer
Theres rubber insulators at the end that goes onto the lower mani that are most likely hard and brittle. While at it, take a look at the knock sensor for sign of black ooze.
 

Salonika

Monterror Pilot
Howah-
Yes, thanks. I never would have believed rubber could turn as hard as the old ones were, but now that I have new ones on there, they are indeed seals! Because the FSM referred to them as "insulators" and not "seals", I figured they were electrical insulators for some reason. I guess 17 years of service will do it.

Now about the knock sensor. I'm not sure if my engine has one. I can only find one reference for it in the FSM and that is one random bullet point in the circuit diagrams. It isn't listed in the index under "sensors" and doesn't show up anywhere else in the index either (unless I'm missing it). I'm fairly familiar with the engine as I've done a lot of work to it and I can't recall seeing a knock sensor. Would the DOHC 3.5 have one? I know I need to start posting the engine code but just haven't gotten around to it yet. Where is this thing hiding? Where does it plug into the harness?
Thanks!
 

Howah

Observer
Knock sensor on my 3.5 dohc is located under the lower intake manifold. In order to take off the the sensor, the lower manifold must come off. Harness is connected on top with the otheres like icm, cam/crank sensor, egr temp sensor.
 

Salonika

Monterror Pilot
I have the 6G74 engine, and I was all over it searching for a knock sensor. Everything is apart except the lower manifold (the one that bolts to the block), and I can see everything, and it's not there. Also I have nothing on my harness. I do not believe I have one. Can anyone confirm this?
Thanks!
 

Salonika

Monterror Pilot
Ok, I wish I could change the name of this thread to "searching for solution to rough idle" because that's what the root problem has been. So a few weeks ago I finished all of the work on the fuel rail and I knew my injectors and seals were all good to go and I still had the rough idle. So I decided to pull off the throttle body and give it a thorough inspection and cleaning. I replaced the IAC seal, the lower bowl gasket, the main throttle body gasket, and a small o-ring on the plastic adjuster screw on the throttle body. This seemed to make the difference as my idle has finally smoothed out. Problem now is my idle is hunting up and down a few hundred RPM in park and neutral, and I had a loud hiss from a vacuum leak. I finally figured out that this hiss is coming from the adjuster screw because when I screw it in far enough and my idle drops, the hiss stops too. The o-ring I replaced was toast, hard as plastic, but I had no hiss before I tore it apart. The air passage was nearly clogged and that is cleaned up now. The o-ring is from Mitsu so not sure what the deal is. Oh and this adjuster screw has a rubber dust plug in the diagram that I have never had. Any ideas out there? Thanks.
 

Salonika

Monterror Pilot
Turns out the adjuster screw just needed to be turned in. This screw is known as the SAS adjuster screw. I had measured the position of the screw prior to removing it and cleaning the passageway. Once it was clean though it was moving way too much air in that position and needed to be re-set. I followed the procedure in the FSM to check the idle speed and adjusted the screw until it was in spec. The screw is now in a completely different position but the engine is back to normal and the idle hunting up and down is gone. But.......the rough idle remains. Damn.
 
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Salonika

Monterror Pilot
Figured I would close this thread out since I finally got the rough idle sorted out. Finally broke down and brought it to the dealer. They performed the IAC recalibration procedure and reset the idle. They also said I put the throttle body gasket on backwards which caused a minor vacuum leak (I can't imagine how but whatever). Truck idles like new now and stalling behavior is gone. This ends a nearly 4 month long saga. Time for a road trip.
 

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