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3.8 liter has no power, knocks, and won't rev past 3000 rpm

  #1  
Old 05-09-2018, 11:27 PM
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Default 3.8 liter has no power, knocks, and won't rev past 3000 rpm

2006 T&C Van 3.9 liter w/ 99k miles

Has been losing power, and won't climb hills. If you give it throttle it downshifts, but won't rev past about 3000rpm, and the sounds are AWFUL with terrible spark knock, shaking, and the engine sounds like it's coming apart. If you rev it in neutral, it will go to about 3500rpm, them start oscillating between 2500 and 3500 rpm with your foot hard on the throttle-feels like someone is moving his foot up/down on the throttle, but it doesn't sound happy.

It idles kind of OK, and if you only use part throttle, it drives OK.

But gradually over the last 500mi it just keeps getting worse and worse.

I had a Check Engine, and finally had it checked, and it said it was a P0404. Tech said I had a sticking EGR valve and tried to free it, but it wouldn't unstick, so he installed a new one.

The engine doesn't run any better, but now I have no Check Engine light. The gas mileage has fallen from 19mpg to about 10mpg. The engine temp is OK.

If I only use a little throttle, it starts and runs kind of OK (not as smooth as in the past). It just won't go up hills, accelerate, or make it past about 55mph.

Tech wants to put it on his lift and check it for a "clogged cat," but I another expert said that if there is no Check Engine, it absolutely cannot have a problem with the cat (or it would give other codes), and maybe the main computer is bad, and it could cost $2000 for a new computer,
 
  #2  
Old 05-10-2018, 03:37 AM
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Hi,
I think you're right, clogged CAT would throw out sensor codes as the two sensors (One before the CAT and one after it) would get wildly different values and generate the fault code.
Now that's not to say there isn't a restriction in the exhaust outlet somehow that's causing this, but that might not be easy to check for.

knocking, shaking, etc, starts to sound like possible timing belt issues, slipping teeth or worse, but you could be onto a huge list of problems, so do the simple ones yourself to be sure.
Checked oil, water, gearbox fluids, etc, all at correct levels (not overfilled with oil).
Clouds of smoke either trying to put your foot down or idling or starting? what colour is the smoke?

All sort of things so go through some easy diagnostics first.
 
  #3  
Old 05-10-2018, 03:38 PM
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Originally Posted by andyb2000 View Post
Hi,
I think you're right, clogged CAT would throw out sensor codes as the two sensors (One before the CAT and one after it) would get wildly different values and generate the fault code.
No O2 sensor DTCs, though the P0404 is back. The old EGR was definitely sticking and would not pop back up. New one shows 0 duty cycle and 0 flow at zero throttle, and up to 75-80% duty at throttle above closed driving. Flow in grams seemed low at 1-4gm, but I don't know what it's supposed to be.

The pre-cat O2 voltage varied fairly rapidly between 0.3 and 0.7v, but the after cat O2 showed 0.75-0.78v and never varied from those values. Super-imposed, the lower post cat and pre-cat varied together, but I would think the post-cat should show more variation than 0.003 volts?


Now that's not to say there isn't a restriction in the exhaust outlet somehow that's causing this, but that might not be easy to check for.

I listened at the back and I can hear engine pulses from individual cylinders firing, and I can detect immediate exhaust volume with the throttle is blipped, not like the cat is an accumulator "holding back" exhaust gasses.

The Cat temp seems high at 1200 deg F at idle seeing the actual EGT at idle is probably only 800 degF

I tried to access the pre-cat O2 sensor (I made up a screw in adapter to use w/ a hose and pressure gauge, but I couldn't access the pre-cat O2 behind the engine w/ the O2 socket I have. Maybe I can find a deep O2 socket and try again?


knocking, shaking, etc, starts to sound like possible timing belt issues, slipping teeth or worse, but you could be onto a huge list of problems, so do the simple ones yourself to be sure.
It's a timing chain engine, and slipping a tooth at only 99k miles seems unusual (not impossible, of course). Not sure how easy it is to check that. Doable, but EASY?

Checked oil, water, gearbox fluids, etc, all at correct levels (not overfilled with oil).
Yep-right-on w/ all.
Clouds of smoke either trying to put your foot down or idling or starting? what colour is the smoke?
No smoke whatsoever visible. Maybe a momentary puff at start-gray?

All sort of things so go through some easy diagnostics first.
It SHOULD be easy. If I can thread my adapter into the pre-cat O2 bung, it will be easier to eliminate the clogged-cat issue.

The symptoms fit the clogged-Cat description perfectly, but who knows?

Bob
 
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