Novak Conversions Jeep Wrangler TJ engine mounts

P0300

Ken G Ford

Ken G Ford

TJ Enthusiast
Original poster
Joined
Apr 29, 2021
Messages
114
Location
Gilbert, AZ
1997 4.0 32RH. Strange issue I am now having...... rebuilt engine, new catalytic converter, plugs, leads, distributor Cap and Rotor along with Switch Plate.

Throws a P0300 ONLY when I accelerate really fast? Driving around on surface streets, no issues at all. Idles smooth as silk, no stumble at all. Just flashes and then throws a P0300 code only when I accelerate hard.

Thinking maybe a vacuum leak leaning out?
 
Could be. Check the rubber “L” fittings on the intake manifold ….kind of low on the side. Exhaust cooks them.
 
Crankshaft position sensor? I believe a new one comes with a cardboard spacer to set it up with the correct distance. Possible it was installed without the spacer and gapped incorrectly.

-Mac
 
NGK brand. Hmm…. Im aware of the paper material on the tip of the sensor that gives it space before it hits flywheel. Couple of rotations and the paper fallaway.

With that in mind, I was a little nerviius about loosening the sensor again but decided to loosen it, push it all the way in, then ever so slightly use the tips of my fingers to pull it out a smidge, like what felt like the thickness of a thin gasket .

Wasn’t expecting any miracles but just took it for a spin. Hammered the throttle up to 70mph 4 or 5 times snd guess what, no P0300!!!! Lets hope it doesn't come back @macleanflood for the win!…. Twice! :)

If anything changes, ill repost but till then, im gonna cross my fingers..
 
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P0300 Back. Comes on intermittently while driving 65mph around 2700rpms to 3000rpms after installing and adjusting the CPS. Put a new Distributor cap, switch plate and a new coil. Still throws a Code when on the hi way at speeds above 65mph. Not right away but only after 20 or 30 miles? Going to throw some XP985 Auto lite plugs in and see what happens although the current plugs are still new. Cant recall the brand? If that doesn't fix the issue, maybe a set of ignition leads.
 
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So it only happens when accelerating. This makes me wonder if it's a wiring harness issue (a break in the wire somewhere) that upon acceleration is causing the wire to arc out on the body or frame somewhere.

Sounds obtuse, but it's always a good idea to rule out the simple things first.
 
Yes, only happens at Hiway speeds around 60mph to 70MPH and either accelerating or abruptly letting off on the accelerator. Gotta figure out how to try and use the data logger.
 
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So, pulled all plugs and found one installed by engine builder. Thought it might be the problem.

IMG_6118.jpeg


IMG_6117.jpeg
 
Installed all new plugs and it ran smoother. Threw a Cap and Rotor at it as well with a new Coil. But still had intermittent PO300 CEL. Tried three different NGK brand CPS. Each time i installed with the new paper distance gauge on the tip, it would idle rough and threw a CEL just going around the block.

Saw a Standard Brand for sale on Amazon that showed the plastic sensor in grey, just like my original that idled smooth but would throw PO300 above 2800 to 3000 RPMs.

Ordered it up and was suprised that it ifled smoothly.

Took it for a drive on the Freeway to re-enact the scenario that would cause intermittent PO300. Several quick accelerations and deceleration on the Freeway. No CEL ! So far so good. Maybe just me but it seems to have a bit more pep and easily pulled up to 78mph which suprised me.

So, does anyone on here think that a new CPS could make such an improvement even when NOT throwing a CEL PO300 ? Or am i just imagining things ? My understanding is that its a Magnet, and sends a 5v trigger to ECM. If would seem to me that it does or doesn't. I could see the 5vdc braking down at higher RPMs but why does it feel smoother below that ?
 
With the quality of parts these days it's anyone's guess.

Nice job chasing that problem down the rabbit hole.

I'd watch that broken spark plug and check it again shortly...might have valve/piston clearance issues...or just a cheap spark plug.

-Mac
 
Yea, firgot the brand but it was the cheapest plug they make for tge 4.0. Engine builder probably saving money. Shame to see that since the engine feels really strong after the zero mile rebuild. Like new engine, which it basically is. I ended up throwing in a set of Autolite XP985
 
Last edited:
You can edit the post and clean it up.

FYSA:

Jerry Bransford

Thread 'What are the best spark plugs for my Jeep Wrangler TJ 4.0?'

This question comes up often, so I figured it's time for a FAQ on the matter.

The later model 4.0 engine (2000 and later) is what is known as a DIS (Distributorless Ignition System) which is far fussier about what spark plugs it likes to run well on. A DIS ignition system also fires the spark plugs twice as often which means a conventional copper-core spark plug only lasts half as long in this type of engine. All 4.0L engines that are 2000 and newer have the DIS ignition system. 1999 and older 4.0L engines aren't nearly as fussy so long as they are the right model plug. 2000 and newer...
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-Mac
 

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