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Discussion Starter #1
98 2.5L 4WD frontier, California emissions model.

Where can I find a replacement exhaust manifold, and possibly the two catalytic converters?

Nissan says they don’t have them any more.


Why am I looking for one:

My exhaust manifold has a crack. There is no noise or serious exhaust leak from it. No exhaust noise and the heat shield is not smoky. Truck runs fine and I get about 22MPG.

However I do have a strong positive Long Term Fuel Trim (LTFT) of +9.8% at idle. I think (but may be totally wrong) that it could be that at idle the crack lets in some fresh air. The air is then detected at the oxygen sensor (manifold crack is 4” upstream from O2 sensor) which is interpreted by the ECU as a lean condition to be compensated by a positive LTFT. This likely results in a rich condition, but since it happens only at idle I don’t notice any effect on MPG. At higher RPM there may be enough exhaust pressure that air from the outside cannot enter the manifold crack, so LTFT at higher RPMs are normal.

I have covered the crack with heat resistant cloth (the stuff they use to protect walls from a propane flame when soldering copper water pipes) and clamped down the cloth wrapped in aluminum flashing. All this to reduce any fresh air entering the manifold through the crack and contaminating the O2 sensor reading.

The problem is that I have noticed a trend of increasing Hydrocarbons detected during our bi-annual California smog test. While the truck still passed last time, the increasing trend indicates that it may not pass in two years. My theory is that this phantom rich condition caused by the exhaust manifold crack is both causing higher HC emission and also accelerating deterioration of the catalytic converters.

The issue would be fixed by replacing the exhaust manifold and/or the cat converters but I cannot find any. Or I could just keep going like this and if I fail the smog test in two years then just clean the cat converters and try again ... etc.

I’m wondering if anyone has some thoughts on this and possibly where to find a replacement exhaust manifold and possibly the cats for a 2.5L ‘98 4WD California Frontier.

Thanks.
 

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NF Mod/Nissan Master Tech
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If it's a '98, then it's a 2.4L (KA24DE engine), not a 2.5L. Go to Rockauto.com. They have both C.A.R.B. compliant and non-C.A.R.B. compliant options. I'm not a big fan of Davico or Pacesetter, but Walker, Bosal and Eastern Catalytic are all pretty good. Check their warranty as they may want proof of upstream oxygen sensor replacement to enforce the warranty if you have any problems.
 

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Discussion Starter #3
Thanks for the reply. Yes, indeed 2.4L engine, my mistake.

I had looked at the options at Rock Auto, but none of the options seem to fulfill the double requirement for California emissions and 4WD. For California I need two catalytic converters, the pre-cat and the rear cat. The pre-cat is built into the exhaust manifold. The Rockauto options imply that the rear cat, and likely the front cat assembly too, are different between the 4WD and RWD models.

So I circled the only options that are available for California (see photo below). Yellow circle for the front cat and manifold, and red circle for rear cat.

1. Front cat and manifold:

For the front cat only the Bosal 0622036 is marked legal for California but it does not say it is for 4WD. I thought that perhaps the front cat / manifold assembly is the same between 4WD and RWD models — but the Pacesetters 757707 and 757697 are specifically marked for 4WD, implying there is a difference between the 4WD and RWD models, except the Pacesetter models are not California legal. So the Bosal 0622036 which does not specify 4WD or RWD might be the only gamble hoping that it fits my 4WD.

2. Rear cat:

For the rear cat as you can see there are only three California compliant options and they are all marked for RWD.

I’m not sure I read things correctly but, bottom line, nothing seems to be available as a rear cat and even for the front cat it’s a gamble whether the Bosal will fit my 4WD.

My HC emissions have steadily increased from about 18 six years ago to 43 on the last smog check and 55 is the CA limit. I’m afraid that if I don’t pass in 2021 my dear trusted Fronty is headed to the junk yard. Seems like scrambling to fix something like this under the impending threat and constraint of an expiring registration would be too much to handle.

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NF Mod/Nissan Master Tech
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Well, the back is not too much of a problem since you cut the flanges off the pipes and install a universal converter that is C.A.R.B. compliant. AP Emissions does make a direct-fit rear converter but the part doesn't appear to be available anywhere. While they are expensive, Magnaflow has front and rear, C.A.R.B. compliant, converters that, according to their catalog, will work on a 1998 Frontier KA24DE 4x4. Magnaflow # 452028-AA is a front manifold/converter assembly that is C.A.R.B. compliant for $855 (Ouch!) and Magnaflow # 457106 is a C.A.R.B. compliant, universal, 3-way converter that should work in the rear for $194:


 

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Discussion Starter #5
Thanks smj999smj. Always helpful. Might be able to address that exhaust manifold crack after all, even if a bit pricey.

A few observations:

One interesting thing about the pre-cat is that the product photo for the magnaflow is exactly the same photo as the Bosal on the RockAuto site which is $300 cheaper. Even the shadows are the same so it is the same photo.
https://www.magnaflow.com/products/452028-catalytic-converter-magnaflow-nissan-frontier-california-grade-carb-compliant-manifold-catalytic-converter?aces=9219566
https://www.rockauto.com/en/catalog/nissan,1998,frontier,2.4l+l4,1316793,exhaust+&+emission,catalytic+converter,5808

As far as CARB, all these products (Bosal at Rockauto and Magnaflow) are all certified for EFN# WNSXT02.423B. The sticker under my truck's hood says WNSXT02.423D. So does the last letter being D vs B matter?

For the manifold+pre-cat neither the Magnaflow or the Bosal specify 4WD or RWD. So perhaps these fit both 4WD and RWD.

These aftermarket exhaust manifolds don't seem to provide attachments for the aluminum heat shield. I don't think I need the heat shield though, as I had operated the truck without the exhaust manifold aluminum heat shield for a few months.

BTW, here is a photo of the crack on my current original exhaust manifold. As I said earlier, it is now covered with heat resistant cloth sandwiched by aluminum flashing and a couple of clamps.

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NF Mod/Nissan Master Tech
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I'm by no means an expert on California emissions regulations. As far as the EFN #, your question would probably be best answered by an inspection shop or even the Dept. of Motor Vehicles in California. The Nissan catalog does have different part #'s for 2WD vs. 4WD, however, it is possible that an aftermarket replacement may be designed to fit both. As far as the pictures, I don't know if they are actual pics of the part or if they are representative? If you have any concerns, you could contact Magnaflow directly and ask them; there should be a contact email and/or phone number at their web site. As far as the crack, if you find an experienced welder, it's possible that they could try and repair the crack in the manifold. It's not easy to weld cast iron, but it can be done. Stick welding with a nickel welding rod is one way and might be the best in your case.
 

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Discussion Starter #7
Thanks for the suggestions. I'll look into welding the manifold crack. Perhaps welding and changing just the rear cat may do the trick for a lot less $.
 
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