Clunk under hard acceleration? - Turbo Dodge Forums : Turbo Dodge Forum for Turbo Mopars, Shelbys, Dodge Daytona, Dodge SRT-4, Chrysler PT Cruiser, Omni and more!

Engine - General, Exhaust & Induction Topics about the general engine items, exhaust system discussed here & Improving the intake tract - air filter to intake valve.

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post #1 of 14 Old 10-19-2017, 04:59 PM Thread Starter
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Clunk under hard acceleration?

Hey all.

We're talking an '89 Lancer Shelby, T2, all stock, 76K. I've been having some heating issues which seem to be taken care of (cooler t-stat, better sealing radiator cap, leakdown test was negative. But a new issue is concerning me.

On long trips, I feel that the engine is occasionally working hard - we can feel it in the pedal, especially when it's on cruise. But what's bothering me the most is a clunk in the engine under hard acceleration at speed, or coming up to highway speed on an onramp. This is intermittent, and I don't have this issue around town. Codes are 12 (car was recently serviced) and 45 (I've checked for vacuum leaks; all good).

I'm wondering if this could be the start of a catalytic converter going out. I noticed ILUVSHELBYS mentioned checking the cat with a laser thermometer; I'll try that and report. But anyone else have an idea?

Thanks!

SBJ

On another issue (the heating) - shouldn't our radiator fans run after the engine's turned off when it's hot?
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post #2 of 14 Old 10-19-2017, 06:26 PM
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Re: Clunk under hard acceleration?

I had a very similar issue with my CSX last year. Check the bobble strut, make sure the nut didn't come off it and it is no longer attached to the bracket. I lost my nut (no pun intended) and at times the piston rod would wedge in the bracket outside the hole and when it found the hole again there was a heavy thud/knock sound and feeling. Sometimes it would get caught in there somehow and I would feel the engine vibration really well through the gas pedal. It was a LOT like what you are describing.

Just a thought.

PS
The laser thermometer trick works really well by the way. If you don't get at least 200 (give or take) degrees difference between in and out after the cat is good and warmed up, then it's not functioning. It's like a furnace, or a blacksmiths hearth, the air passing through it, heats up the catalyst and any air escaping the chamber is super heated.
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post #3 of 14 Old 10-19-2017, 06:50 PM Thread Starter
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Re: Clunk under hard acceleration?

Excuse me for sounding like a rube, but where's the bobble strut? Pretty sure I won't find it marked as such in the Tech manual... :P And if it's the bobble strut (I actually did some research), why wouldn't I get that under normal driving? It only happens at speed, or coming from a hard run (long highway driving). I'm thinking cat still...

And yes, you're the one I was talking about! So, the temp should be less on the outlet side (to the muffler)?

SBJ

Last edited by ShelbyLancia; 10-19-2017 at 06:57 PM. Reason: New stuff
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post #4 of 14 Old 10-19-2017, 08:15 PM
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Re: Clunk under hard acceleration?

The 'bobble' is also known as the torque strut and it is attached to the K-frame and then to a bracket that is bolted to the back side of the transmission. You can only see this from under the car. The bobble strut is going to be just behind the oil pan as viewed from the front of the vehicle. It's just something pretty easy to check, as I think I mentioned, mine had a very similar symptom.

On the cat temp, it should be hotter on the outlet, the end connected to the muffler. Engine exhaust enters the front, it gets the furnace action going in the cat, and when it exits the cat, the air is much hotter. So air coming out is hotter than air going in.

Here is what the bobble looks like...it is pictured upside down. The nut on top is not a joy to work on BTW.

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post #5 of 14 Old 10-20-2017, 05:27 AM
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Re: Clunk under hard acceleration?

Quote:
Originally Posted by ShelbyLancia View Post
On another issue (the heating) - shouldn't our radiator fans run after the engine's turned off when it's hot?
No, that was only on early L-Body cars.

If you want to know if you are running at the correct temp backprobe the CTS with a digital voltmeter and read the voltage.

Also, do not forget that a Restricted Exhaust, Advanced Cam/Ignition Timing will also cause higher engine temps.

Clunk under hard acceleration?-backprobing-cts.jpg

Clunk under hard acceleration?-cts-voltage-chart-2.jpg
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post #6 of 14 Old 10-20-2017, 10:44 AM
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Re: Clunk under hard acceleration?

While you have your laser thermometer out, check your front rotor temps after driving. You wouldn't be the first guy to feel engine laboring due to front brakes not fully releasing.
A restricted cat or bad muffler will definitely kill a bunch of HP. A lot of the time, the cat will break apart or melt internally and it will sound like loose heat shields rattling under car.
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post #7 of 14 Old 10-20-2017, 01:04 PM Thread Starter
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Re: Clunk under hard acceleration?

Thanks for the info, folks. Didn't have my SL long enough to delve into such stuff, and the whole fiasco with the pistons (magnesium; they deformed) sort of kept long distance drives to a minimum.

UPDATE: I just checked the exhaust temp in/out of the cat - only a 100 degree difference.
Sounds like I have an issue...

Last edited by ShelbyLancia; 10-20-2017 at 04:48 PM. Reason: Update
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post #8 of 14 Old 10-20-2017, 08:12 PM
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Re: Clunk under hard acceleration?

If you want to verify an exhaust restriction then you need to check backpressure.

Your symptoms of the car laboring and loss of power after running for awhile also sound like the classic low fuel pressure symptoms.
Hook up a gauge, get your baseline static, idle and under boost pressures and then go for a ride and wait for the symptoms to appear.

CHECKING FUEL PRESSURE
https://www.turbododge.com/forums/f4/...-93-turbo.html
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post #9 of 14 Old 10-21-2017, 07:33 AM
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Re: Clunk under hard acceleration?

Quote:
Originally Posted by 4 L-bodies View Post
While you have your laser thermometer out, check your front rotor temps after driving. You wouldn't be the first guy to feel engine laboring due to front brakes not fully releasing.
A restricted cat or bad muffler will definitely kill a bunch of HP. A lot of the time, the cat will break apart or melt internally and it will sound like loose heat shields rattling under car.
I have seen front brake hoses squeezed shut by rust accumulating between the hose and bracket.
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post #10 of 14 Old 10-21-2017, 05:12 PM Thread Starter
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Re: Clunk under hard acceleration?

UPDATE:

The Bobble Strut is fine and solid.
As mentioned before, the temperature diff on either side of the catalytic converter is 100 degrees - too little? No rattle underneath.
The brakes do not appear to be sticking - temps are ok and no glow is seen.
I haven't been able to check the fuel pressure.
Took it out for a drive today, and the clunking now happens more regularly, but only at highway speeds, always under a strong acceleration.

SBJ
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post #11 of 14 Old 10-21-2017, 05:20 PM
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Re: Clunk under hard acceleration?

Check the front motor mount bracket, they sometimes break for fatigue or from the bolts working loose, also check on all your motor mount rubber. The front one wears out on a regular basis if driven hard and the drivers side tranny mount can completely come apart leavng the bolt actually resting on the mount flange. The passenger side is a lot easier to see if it is toast or not. Check also the bolt on the front mount to radiator support/lower rail, if it is a bit loose it could make a thud noise under load or when getting off load. Just a thought...
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post #12 of 14 Old 10-21-2017, 09:03 PM Thread Starter
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Re: Clunk under hard acceleration?

I'll check the brackets and mounts, but this feels more engine related. It feels like the engine is seizing.
I think low fuel pressure or a clogged cat sounds more possible, IMHO...

SBJ
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post #13 of 14 Old 10-21-2017, 09:55 PM
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Re: Clunk under hard acceleration?

Well it could be any number of things, I am though trying to help with the 'clunk' sound that you've described.

Back on the cat trying to be stopped up, if you can get the oxygen sensor out of it, then install a back pressure meter/gauge in it's place, then the reading at idle should be 1psi or less, if you rev the engine to something like 300-4000RPM then it should climb a bit but not exceed 3-4 psi max. 3 would be ideal. If it's 4 or more there could be a restriction in the exhaust somewhere, in cluding the cat, pipes or rear muffler. A temperature difference of 100*F from inlet to outlet is in my shop at the low end of the pass scale. I would like to see it closer to 150 or as much as 200* more if it was really doing its job. Being a bit low however is not a clear indication of it also being clogged.
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post #14 of 14 Old 10-22-2017, 07:19 PM Thread Starter
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Re: Clunk under hard acceleration?

ILIKESHELBYS:

Thanks for the help. I'm really appreciating it. I'll check them out!

SBJ
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