# Exhaust Manifold Crack – Messing Up O2 Readings?



## 98Frontier4WD (Jan 12, 2015)

My truck is idling and running well again  after the last bad MAFS episode. 

However, I noticed that the idle LTFT (Long Term Fuel Trim) has settled into a rather strong +9.38% and that the STFT (short term fuel trim) is often positive at idle (+6 to +9%), especially when I let the engine warm up idling but do not quite drive the truck yet.

So that is a total (STFT+LTFT) positive trim of about +15% which seemed a little too high. Also at higher RPM and other engine loads, LTFT is closer to 0% and STFT hovers around 0% as it should.

Having sought vacuum leaks recently, as part of my rough idle diagnostics (which, as noted above, turned out to be a bad MAFS), but having found no vacuum leaks whatsoever, I decided to concentrate on a small crack I have had in my exhaust manifold. I’ve had this crack for a few years now.

You can see the crack here, on the exhaust branch for cylinders 1 &2.





So, I thought…

Could perhaps O2 enter through the crack (normal flow should be out, but I read there are also short periods of negative pressure in the exhaust manifold, so air could be sucked in and cause the primary O2 sensor to erroneously report lean condition—thus prompting the ECM to increase fuel trims? 

Seems also that the ECM is suspicious of something, because it does not transfer the STFT positive bias into the LTFT, no matter how much I let it idle. 

So I decided to do an experiment: Temporarily plug the exhaust manifold crack with something, and see how the STFT responds. 

So I plugged in my OBD2 dongle (best thing I ever bought for $20) and using my iPhone and the OBDCarDoctor app (another great thing for a mere $4) I recorded the STFT values into a text file (the app then lets you email that text file to yourself) while I plugged the manifold crack with some crumpled up aluminum foil and a piece of fiberglass to apply some temporary pressure with my gloved hand. Temporarily, just for a few seconds, just to see if STFT responded.
I also started a voice recording on my iPhone making comments as to what I was doing. Then I can correlate the voice recording with the OBD2 text data since the data is time stamped. All you have to do is say “starting recording now” on the audio as soon as you press the record button on the OBDCarDoctor app. Then you know which exact second in the recording matches the time stamp for your first OBD2 sample, and then from there you can correlate all your audio comments with the myriad time stamped OBD2 samples. I used Microsoft Excel to post process the OBD2 data and create a graph. If you know some spreadsheet basics and graphing, there are many ways to do that in a spreadsheet using the OBD2 data. If anyone is interested and needs help I can describe the process later.

Anyway, back to my measurements:

By golly! STFT trended close to 0% when I plugged the crack. Here is how I plugged the crack:
Put a piece of crumpled aluminum foil around,

Then used a piece of fiberglass and my gloved hand to press on it.








And here is what I got as an STFT trace! I put the comments I made on the audio recording on the graph. As you can see, as I closed the aluminum foil onto the crack, without even pressing on it, the STFT started going down. In disbelief, I repeated the test later (no recording) just watching STFT in real time. Same result, STFT dropped as soon as I plugged the crack. How much can I plug an exhaust manifold crack with a piece of alum foil being pressed essentially from one side only? Yet it seems enough to significantly reduce the air that is apparently being sucked into the crack – which then ends up on the O2 sensor and tricks the ECU into erroneously increasing trims?



Another thing that may be related to all this, is that my secondary O2 is showing lean (0.06-0.09V) during this idling. Of course I’m a little puzzled by the location of my secondary O2. Seems like my truck (being a California edition) has two catalytic converters? One (I assume the pre-cat) is what you see in the photos above, then there is another one under the vehicle, behind the driver seat. The secondary O2 is then between the pre-cat and this second cat. So I’m not sure what it should read, though I did make a recording of its values during a drive a few days ago. But seems premature to talk about that. 

So is it possible that: 

a)	ECM sees primary O2 reporting presence of O2 (entering from the crack) and thus ECM erroneously puts more fuel, resulting in both HC from rich combustion and O2 coming in from exhaust manifold crack being dumped into the pre-cat…
b)	But Pre-Cat now finds itself with more HC and at the same time more O2 than normal. And is thus unable to digest them both?...
c)	Hence dumping into the secondary O2 both more HC and more O2 than normal. Secondary O2 cannot pick up extra HC but does pick up extra O2 and reports lean? When it reality combustion is rich?

See also that ramping that STFT seems to be doing in the graph? Going methodically between about +6% and +9%? That is too slow to be the normal rich-lean oscillation, as the ramp seems to last about 5sec. Perhaps the ECM sees something inconsistent, perhaps it sees that in spite of its increased STFT, the secondary O2 is not going rich, and so it keeps hunting some condition? Perhaps this inconsistency makes the ECM decide to not transfer the STFT bias into the LTFT?

…well seems like this is a lot of speculation … but one thing seems quite likely: There seems to be O2 entering the exhaust manifold and contaminating the O2 signal. 

So the next issue is how to fix it. Preliminary Google search was not too promising. Seems like these California exhaust manifolds with the pre-cat are not available anywhere? 

So perhaps I can seal the crack with some hi temperature glue/putty? I’m reading mixed reviews on how effective those are. In my case the crack is all the way around, I doubt the glue would hold the stress of what is likely a moving crack. Of course all I need is something that simply blocks most of the air. As the aluminum foil shows it may not take much to block the air.

I suspect that this California exception might have been done by Nissan in a hurry and that these exhaust manifolds are congenitally stressed beyond their limit and crack. I certainly never messed around with it. 

So is the crack the culprit here? And how to fix it?


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> Could perhaps O2 enter through the crack (normal flow should be out, but I read there are also short periods of negative pressure in the exhaust manifold, so air could be sucked in and cause the primary O2 sensor to erroneously report lean condition—thus prompting the ECM to increase fuel trims?


Yep, same pulses that pulse at the MAFS, also pulse at the exhaust. But it's more the velocity of the exhaust gasses that cause the lower the ambient air pressure rather than the exhaust gasses themselves being lower than ambient pressure.



> So is it possible that:
> 
> a)	ECM sees primary O2 reporting presence of O2 (entering from the crack) and thus ECM erroneously puts more fuel, resulting in both HC from rich combustion and O2 coming in from exhaust manifold crack being dumped into the pre-cat…
> b)	But Pre-Cat now finds itself with more HC and at the same time more O2 than normal. And is thus unable to digest them both?...
> ...


Cat's load up after a bit, e.g. they get "filled" and can't process any more of the unburned gasses. The ECM magically sees that and tries to get the cat to "unload" after a bit.
I can see that being the cause for the slow rich-lean thing going on.

Exhaust manifolds can be welded, by a competent welding professional.
I got a Dodge 440 in a Class C motor home. Now you know that thing gets hot! Cracks all over the place in both exhaust manifolds, both near the exhaust ports and down by the collectors. And of course, damn near impossible to find the proper replacement, mainly because it's a motor home and the manifold is a bit weird in shape.
Had them welded up back in 2010. Been holding tight ever since. Don't remember how much it cost, but couldn't have been too much, otherwise I would've remembered that particular information.


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