1986 3.8L Slight Knock?

[sreilly77]

I have a 1986 Thunderbird, 38,000 miles, 3.8L. It has a slight knock/tap/rattle under low RPMs only when in gear. If you rev the engine in park, its smooth as butter. But when in gear and around 1,500 RPMs while the car is in motion, there is a slight knock/tap/rattle (hard to describe). Oil is good, coolant level is perfect, no indication of head gasket issues either. I'm suspecting a possible exhaust leak. However, it is pretty hard to isolate since the sound is only made while in gear and the car is in motion.

At idle, it's runs quietly and smoothly, regardless if it's in gear or park. Anyone have similar issues, or wonder what it could be? I find it strange that the noise only occurs when the vehicle is in motion around 1,500 RPMs. As the RPMs increase beyond this point, the noise goes away. And once again, when it returns to idle, the noise goes away as well.

Any help would be greatly appreciated!

[65comet]

I would put in a mechanical oil pressure gauge and see what the pressure reads when the condition occurs. Most Ford oil gauges in the dash are really just a modified idiot light and don't show a real pressure reading.

[sreilly77]

Originally Posted by 65comet

I would put in a mechanical oil pressure gauge and see what the pressure reads when the condition occurs. Most Ford oil gauges in the dash are really just a modified idiot light and don't show a real pressure reading.

Yeah this dash only has the idiot light. I'll grab one from the auto parts store and hook it up.

I did some more research and I found out sometimes this motor will have these symptoms when the EGR goes bad, clogs up, or is stuck closed. So I'm going to check into that as well. Being that the car only has 38,000 miles, I'm wondering if there's just a ton of carbon build up from never really going on the highway and getting opened up EVER. I know it was maintained religiously. Anyway, we'll see what happens with the oil pressure gauge.

[sreilly77]

Originally Posted by 65comet

I would put in a mechanical oil pressure gauge and see what the pressure reads when the condition occurs. Most Ford oil gauges in the dash are really just a modified idiot light and don't show a real pressure reading.

So I have been able to isolate the sound from under the car at the rear of the engine, which leads me to believe that it must be an exhaust leak that only happens when the engine is under load. I am also going to check the motor mounts as well. I'm wondering if there is a little bit of torque on the motor that is either 1. Stressing the exhaust out, or 2. causing the oil to slosh around. Being that the motor mounts are probably original, they probably need to be replaced. The exhaust may just need to be replaced as well, since everything from the cats upstream is original.