Possible exhaust restriction?

[BrokeAsFix]

Hey team, I’ve been down a rabbit hole trying to resolve a rough idle and hesitation:
-compression test 145-150 on all cylinders
-New spark plugs leads and coils
-New valve stem covers
-Ultrasonic clean for the injectors and new O rings (this is the step that finally resolved the idle and hesitation, but at 300k I’m sure all the other bits needed doing anyway.)

Now enjoying my freedom I took it up the freeway into the Adelaide hills but going uphill on the freeway felt like I was towing a heavy trailer, with the engine struggling and shuddering at 2-3k rpm and I needed to manually shift down into third gear to get the revs up to 4k so I could toodle up the hill at 80km/h.

Feels like it could be a exhaust restriction but a vacuum test did NOT show a gradual reduction in vacuum with a sustained increase in the throttle.

Does an exhaust restriction always show a detectable vacuum issue? Should I rule out an exhaust restriction, or look deeper?

(2000 NM petrol 3.5)


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[Keithyv]

Exhaust restriction will feel like the engine just hits a wall, it won't cause shuddering in my opinion.

[BrokeAsFix]

Thanks for the reply. The rough idle is gone (fuel injectors) and the shudder is the kind of shudder you get when you have low rpm in a high gear, telling you it’s time to shift down. That kind of thing.

Rpms when in neutral don’t seem to have restriction, and it quickly throttles up when not under load, if that helps?


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[mrbitchi]

It COULD be exhaust. Had similar symptoms on my old 5LT Statesman a few years back. Wouldn't rev, wouldn't change up under load, struggled on steep hills. Turned out the Cat Convertor was stuffed. Almost completely melted internally. New exhaust, different car

[Keithyv]

Yes had a similar issue with Mazda Tribute (melted cat) but it just 'Hit a wall' as I described earlier. Did not run rough or anything like that. Basically completely normal until it needed to make beyond a certain amount of power.

[erad]

I always think the worst, and in this case I have had "the wall" before in my Range Rover.

Are you losing coolant? If you are, or there is too much coolant in the overflow bottle, then you may have a blown head gasket. I had good compressions in the Rangie's cylinders, but when the temperature gauge got to a certain level, I could feel a sudden drop in power. What was happening was there was a very slight leak in the gasket, and when the engine got hot enough, the aluminium of the heads expanded sufficiently to allow leakage to occur to the adjacent cylinder. Of course, there was no way to test the compressions at that temperature, and when they were tested, all showed up fine. One day I noted the coolant overflow bottle was full, and (the engine was cold)I opened the radiator cap and found low level in the radiator.

That made me suspect a head gasket, but which one? In my case, I took a gamble and removed the left head (it was the most awkward one to get at) and saw a slight tracking between No 6 and No 8 cylinders, so I had the head shaved minimally and replaced it with a new gasket. All good after that so I didn't touch the Right head at all.

I hope for your sake this is not your problem.


For what it is worth, your problem may be as simple as a blocked fuel filter, causing fuel starvation when under sustained maximum load. This would make the mixtures leaner and the Oxygen sensor would try to compensate for it but if there was not enough fuel pressure, it would be too lean and you could get breakdown of the high tension leads under these conditions. Get the car in a totally dark environment and lift the bonnet and run the engine. If the leads are shorting to earth, you will see them like Christmas trees in the dark.

[BrokeAsFix]

Fingers cross it is not a cracked head. I’m not seeing signs that it is temperature related. Hot or cold up this hill, it is still problematic.

A melted cat could be it. The engine had a misfire for a long time before I got it, and that can reportedly cause the cat to clog/melt/degrade.

Gonna see if I can find a reasonable method to check the exhaust back pressure before and after the cat/s.


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[Lance]

Could you drop the Cat and go for a run around the block ?

[Scooby]

You can see into the front of the cat if you disconnect at the joint going into it.

[BrokeAsFix]

Thanks, I’ll have a look this weekend. Fingers crossed!


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[BrokeAsFix]

Update to this, it's not the cat or an exhaust restriction.

Took it to an exhaust specialist and while it was up on the hoist we all laughed at how crazy hot the cat was, smoking hot. Seemed like a clear case of a melted cat (the car had been misfiring for a long time before I got it, so this was not a surprise).

Removing the cat, it was in perfect shape inside, so we took it for a test drive with the cat off (no exhaust) and still had the same very sluggish performance. Wasn't able to take it up the freeway (it was WAY to loud to continue driving around town).

So now I'm back to head scratching. What would cause a superheated exhaust without an exhaust restriction or melted cat?

[Two Emms]

Not familiar with the 3.5 petrol but something is making me think it's a timing issue.

I have struck something similar in the past, I think it was a Ford. Simply, the exhaust valve was opening before combustion was complete. Loud and hot.

And lacklustre performance under load

[erad]

Fuel starvation (making the air/fuel mixture to lean) will cause the engine to run hot. You won't notice it on the OEM temperature gauge unless it is really bad, but lean mixtures make the exhaust gases very hot. My old Triumph 2000 had adjustable carburettors, and when I came out of the high country ai used to lean the carbies off to improve fuel economy. One day the engine seemed sluggish so I leaned it off a bit more, and after a while it was still sluggish. I opened the bonnet ands saw the exhaust manifold glowing red hot! OOPS! Your problem could be either the fuel filter blocked or the oxygen sensor not reading correctly, making the mixtures too lean. My bet is the fuel filter blocked.

[stumagoo]

sounds a bit like my NL used to run --- I found in the end it was a leaking gasket between the lower plenum and upper --- the metal one that is - I had replaced it when I got the engine originally and did the spark plugs - I had replaced it a few months after that as I was worried I had done something wrong ---- and when I got the current wagon and dropped the engine in I replaced the plugs again and thistime I glued the gasket down and that stopped it.... I could not get any sign of vacuum leaks the normal ways but I am certain there was one in the end

my other symptom not mentioned here is it idled like it had a mild cam grind