Knock/Rattle at idle when in drive

[6spdav6]

I just joined the forum and am looking for some advice. So far you guys have given me a lot of useful information.

The back story: I just baught my first Ranger, a 90 4.0 auto. Original owner passed away, and I purchased it from her daugher. The truck ran well when I looked at it, and test drove it. The truck came with a 3 inch binder full of it's maintenance records over its life, so I know it was well taken care of, but not driven much over the last 2 years of it's life. After doing the transfer, I drove it home in LA traffic, about 50 miles of stop and go, no problems, and the truck did not overheat. Once I got it home, i changed the oil and filter.

Here is my issue: About a week after I bought it, after about 30 miles on the freeway at about 70mph the whole way, I exsited and stopped at a light. At that time I heard what seemed like a knock. The truck idles and drives fine, but when in gear, with my foot on the brake, I hear the knock.

I have searched the forum, and have read about lifters, mounts, and idler pully bearings. Here is my problem, since I only hear the "knock" with the truck in drive and my foot on the brake, I had by wife hold the brake with the truck running to see if I could better tell where the sound was coming from. It does not seem to be in the engine compartment, and if anything may have been coming from under the cab. The sound is not noticable when in park or neutral, and is also not present when driving. I did notice when she shifted into drive that the motor did torque to the right slightly. I do notice a slight pulsing dimming in the headlights when they are on and the truck is held at a stop in drive. Again, not when driving or in park.

Sorry for the long post, but I wanted to give as much info as possible. Hopeing someone here has dealt with something similar and can help point me in the right direction. Truck is stock supercab, 2wd, 4 speed auto 4.0 v6. The trans was replaced with a new factory one about 2 years ago, and does not have many miles on it. I am hoping it is something simple like a worn out mount, but have not been able to pinpoint where it is coming from yet.

Thanks in advance for any help someone can give.

 

[RonD]

Here is the info on the tranny
http://www.therangerstation.com/tech_library/a4ld.html

Have you checked the transmission fluid, does it look good and doesn't have a burnt smell.

I use a wooden broom handle, without the broom attached , as a stethoscope, it can be placed in hard to get to places to pin point a noise.

Does it make the noise in R and D or just D?

It is normal for the engine to shift slightly when going from P to D or P to R, etc....
With engine on and in D, foot on brake and hood open , give it a little gas and see if engine lifts, try same thing in R, worn mounts will show up as a lot of movement.
Might also hear the knock louder if it is a contact knock and not an internal knock.

Lights dimming at idle is not abnormal, could be older battery, below 12.4volts.
Alternators don't put out as much power at idle so battery takes over.

Battery voltage should be 12.6vDC or higher with engine off.
Alternator test:
With engine on and at 1,000 RPM voltage at battery should read 14+vDC
Turn on lights, heater fan(hi), inside light, radio and anything else that draws power
Put engine at 1,000 RPM again, voltage should still be 14vDC or higher

 

[Explorin94]

Sounds like you need to look for a broken motor mount or transmission mount. The lifter tick is just that a tick. It is very noticable. As for the lights dimming check your charging system and go from there.

 

[6spdav6]

Thanks for the advise guys. I never thought of using a broomstick as a way of helping to locate a noise, but I like the tip. The noise is present in both R and D/OD.

Trans fluid looks good, no burnt smell. The truck does have a slight oil burning smell after running for a while, but I'm pretty sure its the valve cover seals. Haven't had a chance to poke around much more, but am planning on it this weekend.

Thanks again. I'll let you know if I find anything.

 

[6spdav6]

Sorry it has taken me so long to get an update on this. I had a friend of mine who used to work at a Ford Dealership look at it with me the other day. It took a while to pin point the noise, but finally figured out it was coming from the passenger side valve cover. When he took the oil filler cap off, he said you could hear it better from the area as well (I was inside standing on the brake). We used a screwdriver as a stethoscope to help find it. His guess was that it was either a bent valve or sticking lifter, leaning toward lifter since it was not present all the time. He said he thought that maybe something had come loose in the oil during when I changed it and that maybe that had clogged it enough that it would be noticeable at idle, but only in gear because it had slight load on the engine. Do you guys thing this sounds feasible.