Failed smog - High HC, High NO

Took my 87 Dakota in and it failed 4 times. High HC and high NO. And while the failure points to a bad EGR system, I am absolutely certain the EGR system is functioning as it should. The whole ignition system is completely new. The carburetor is in great shape. The timing is retarded 3 degrees from specs. The truck has passed with this exact same configuration since 2008. The engine is the 3.9L V6 with 170,000 miles, auto trans. Drives like a dream and the AC works!

The only other thing that I can think of besides a bad EGR system is a lean condition that results in lean misfire. The only possible solution that I can think of is to put in larger main jets. The mechanical fuel pump appears to be working. The carb bowl fills when the engine is cranked and the floats shuts off the fuel. Float is adjusted properly. Other than changing the main jets I am flat out of ideas so I thought I would put my line in Car Talk waters to see if I might catch some insight.

I might add that the cat tested good by using an infrared thermometer to read the exhaust pipe going in and coming out, over 100 degrees F. hotter coming out. It is a California approved replacement installed about 4 years ago.

I might also add that manifold vacuum reads 19 to 20 at idle.

Have you checked for a stretched timing chain?


I did not check for a stretched timing chain. I would have took up how that is done.

Did you take it for a long drive before the test? My state recommends at least 20 minutes of highway driving before testing.

I drove it on the freeway for about ten minutes, then streets for another 5 minutes. Left it idling before the test, which was perform right away, as there was no one ahead of me.

It wouldn’t be hard to do, and I probably will do it, but the truck runs like a dream. Starts up in 2 seconds and accelerates with lots of power at all speeds.

Remove the distributor cap.

Get on the crank bolt with a socket and breaker bar.

Turn the engine over in it’s normal direction while watching the rotor in the distributor.

Now turn the engine over in the opposite direction while watching the rotor.

If the engine can be rotated in the opposite direction 5 degrees or more before the rotor begins to move, the timing chain is stretched. Which causes the ignition/valve timing to be off causing higher emissions.


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Do you know which emissions would likely be higher as a result of a stretched timing chain?


The ones the vehicle failed.


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My 1993 Caprice would fail emissions if the cat was not fully warmed up. I used to drive at least 30 minutes before I took it to the Inspection Station and even then I would have to keep my fingers crossed I didn’t have to wait too long in line. Any chance of an emissions waiver due to its age? New Jersey no longer tests OBD-I equipped cars so I’m off the hook.

Ed B.

I have an '87 Toyota pickup that passes emissions tests easily when it’s in proper repair (the choke wasn’t working once, the carburetor the second time). In NM only much older cars get waivers. I always drive around for an hour or so beforehand, especially a fast stretch, and I put in a bottle of oxygenator to improve burn.

Near as I can determine using the timing marks and the crankshaft pulley, there is 7 degrees of play between the engine and the rotor.

You just confirmed the timing chain is stretched.



Some faulty conditions will affect certain gases and not others. A rich mix can cause HC and CO to increase, but will tend to cause NOx to decrease. Advanced timing can cause HC and NOx to increase, but not CO. A lean mix can cause an increase in HC and NOx, but will cause CO to decrease. So it is natural to assume that a stretched timing belt sets up specific conditions that that affect the gases that need to pass a 3-gas emissions test in various ways, raising, lowering or no change. To automatically assign blame for the failed gases on a stretched timing belt simply because they failed seems kind of blunt, and not very insightful. However, I do appreciate that you brought up checking the play in the timing belt, and even took the trouble to explain how it is done.

I might also state again that the engine exhibits absolutely no signs of a timing problem. Starts up very quick, accelerates smooth and strong, and idles steadily at the specified rpm.

Yes, I realize that. I am not yet convinced, however, that this is what is causing the high NOx and high HC, and I won’t be convinced until I can find an explanation. NOx, for instance, is generally caused by combustion chamber temps above 2500 degrees, or so I am led to believe. What I would like to know is how exactly does a stretched timing belt cause high combustion chamber temps. Now I don’t expect you to explain it me, even if you can, you have given me enough of your time, and I am perfectly capable of taking the ball you have passed and running with it.


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Are you sure that the EGR valve is working properly? Can you see the pintle move when you load the engine?

I think it was VERY insightful considering… You failed HC and NOX, @tester told you to look for a stretched chain because you failed HC and NOX… and Lo and Behold, you have a stretched chain. So prove him right or wrong by fixing it and re-testing the truck.

You can come back and gloat if he was wrong. But I’ll bet he isn’t…


Myself I observed the consequences of the stretched timing chains on my older car: it was definite wear marks on the trailing side of the camshaft lobes where the stretched chains were “bouncing” abruptly as shaft was moving past the fully open position and stretched chain would let go and allow valve to close earlier than needed.
I would assume this could definitely change the right burn process, as the timing was not uniformly shifted before/after, but the entire open-valve time was definitely below the design specs.

I am convinced that the EGR circuit is performing as it should. I have a vacuum meter inside the cab that is teed into the EGR vacuum line. The readings are good, no vacuum at idle and vacuum begins at off-idle and steadily increases until the auto trans shifts to second, at which point it drops and begins increasing again. I duplicated the speeds and loads used by the dynamometer, 1300 rpms at 15 and 25 mph, by using the break pedal, speedometer and a tachometer. Keeping at a steady 25 mph, apply brakes until rpms reach 1300, read vacuum. The vacuum noted is more than enough to open the EGR valve. I removed the EGR valve (fairly new) to bench test. It responds to vacuum according to the Chrysler shop manual. The EGR valve is not clogged, neither are the passages to the intake manifold and the exhaust manifold, if I can believe the wire that I poked into both ports, which extended to under the carb, and 6 inches or more into the exhaust manifold.

Yeah, the EGR system works, which only leaves me with a very lean air/fuel mix to explain such high NOx… unless there is another explanation that I am not aware of. I have owned this truck since it was new, and nursed it through every smog test, and haven’t found any explanations for such high NOx readings other than faulty EGR system, and super lean air/fuel mix.

It seems that, with a stretched chain, the valves would be slow to close on the compression stroke, causing low compression when the cylinder approaches the top of the compression stroke. I can’t quite see how this would cause higher than normal combustion temps that cause high NOx formation. Of course, the valves would be slow to open on the exhaust stroke, but they would be open for the same amount of time, so the exhaust gases would be allowed to escape. And since exhaust gases are re-introduced by the EGR valve to cool the combustion chamber, it should not be a problem, even if more remain than usual remain in the cylinder because of a stretched timing chain.

I am focusing in on NOx, because I cannot see how a stretched timing can cause high NOx readings, and I would like to rule that out as a cause.

If anyone can explain exactly how a stretched timing chain can cause high NOx, I would be more than glad to read it, but so far I haven’t seen that.