New to me 1990 Grand Marquis Shudder

[1990Merc]

New to the Grand Marquis scene and the forum. I recently bought a 1990 Grand Marquis for $750. Body is pretty good, minor rust, paint wear issues. Engine seems mechanically sound but I've certainly not had it apart or seen whats sitting in the pan yet. The interior is fantastic, better than my 2007 Focus. Anyway, long been an admirer of the boxes, particularly the Crown Vic but with the Grand Marquis I get to say I drive a Merc at black tie events.

I already have new plug wires, plugs, cap, rotor and a brass TV cable bushing on the way although the cable is still connected. I've started to notice a bit of a shudder in the higher gears above 30mph. Hard to describe. After doing some reading here and other places, sounds like it could be a misfire so hopefully when I do the tuneup it will help... But I am a bit concerned about the transmission. Maybe I'm just looking for problems but the transmission fluid is a bit dark, not really the same smell as brand new but I personally wouldn't call it burnt (but I'm not that experienced judging auto fluid). I'm always terrified of automatics dying on me. I normally drive manuals and don't have a whole lot of long term experience with autos as odd as that may sound. Is it worth risking a drain filter and fill on it to head off future problems or am I just going to kill it? If the tranny is dying, I'd like to fix it. I can pull it myself and either have it rebuilt or try working on it myself. The car is not my daily driver and I'm no stranger to working on cars and engines although I've never cracked open an auto.

The question is then, do I let it go or try preventative maintenance? I should also add that the car has (probably) 136,000 miles on it and I would not be surprised if the fluid is factory fill.

If anybody has any other recommendations of things to check on a recently acquired Grand Marquis or other things that might relate to the shudder I'd definitely appreciate suggestions.

Thanks!

[miamibob]

I'd do a COMPLETE tune up so include air and especially fuel filter!! Bobby

P.S. Hoses and belts also.

[sly]

AOD won't have the torque converter shudder that newer cars have because it's built different and can't do that. Misfire is the most common issue for shudder on an AOD equipped vehicle.

[GM_Guy]

Can you get more specific about your shudder?
Does it happen during steady cruising conditions? Does it seem to only happen between 35-45mph? Are you doing around town driving in OD or D?

Alex.

[1990Merc]

It happens in steady cruise as well as moderate acceleration. Seems to clear up when wide open. Normally in D in the city but it does not appear to make any difference whether in D or OD and I don't notice it until moving at least 30mph and up. I'll need to drive it a bit more to come up with a better description and that may take me a day or two before I get the chance. The car is stored about 20mi from my house at the moment.

[1990Merc]

I just remembered something that might be relevant. Off idle torque seems to be pretty good. In the drizzle I caught myself spinning the rears pulling away from stop signs. Rear tread is good but I should check pressures. Transmission is definitely not slipping in 1st and at those low speeds I never noticed any shudder.

I am curious how a dying AOD presents. Just slipping in gear, revs rising without acceleration or will the clutches chatter or cause any sort of shudder? I've never driven a bad auto before (that I know of). I know what a bad manual transmission clutch feels like. Is it similar?

[Louis]

Could be as simple as bad spark wires....once you change those you should notice a big difference in overall performance including trans performance.

If you do decide to change the trans fluid later, just go with a pan drop and 4-5 qt. refill (and change the filter) as opposed to any type of 'power flush' which could do more harm than good.

[BerniniCaCO3]

"Does it seem to only happen between 35-45mph? Are you doing around town driving in OD or D?"
He's getting at the hard-ish shift into overdrive. This is normal/ not a problem/ only some serious mods make it go away (like a different torque converter from dirtydog).
It isn't a bad idea though to keep it in drive around town.

Where I used to live, all the streets I drove on were right at 40mph speed limits, and my car would go into and out of overdrive all the time. Annoying, and not great for the tranny. It would shudder every (upshfit?). So I forced it into D instead of OD-- much better.
For fun, I later took apart the transmission to beef up. I can state that it was perfectly healthy, so the shift into OD is not a symptom of a failing trans. Just built into the design. It's been almost 2 years since I was into this... there's this shaft that runs through the transmission and I think OD and maybe 3rd bypass the torque converter somehow using this, for a simulation of a locking converter before locking converters were a thing (please correct me). This gives better highway fuel economy, but gives that lurch. So you can get a modified non-lockup converter. You can also move your shift points up with a valve body kit or --within limits-- tweaking the TV cable, so it shifts into overdrive at a higher speed than 40mph, so it isn't always shifting up and down at what is a very common in-town speed limit.


I did notice when I first bought the car a bit of a shudder/ lurch on the highway which turned out to be an ancient and long overdue fuel filter that was starving the engine, causing misfires. So that's another option-- along with just plugs/wires. They're not hard, or expensive, so just do them

[1990Merc]

Update:

Changed cap, rotor, plugs and wires tonight. Much better. The #8 plug was gas soaked but otherwise didn't look too bad. My guess is the plug wires were bad. Anyway, the problem has mostly or completely gone away. After about 30 miles of driving I'm quite pleased. Also filled up the tank and added a bottle of techron in the hopes that it might do some good for the injectors. Did not get to the fuel filter tonight, I might get that done tomorrow or next week, along with a drain and fill of the transmission which I've decided to risk.

Thanks for the advice everybody

[91GMEric]

Wouldnt hurt to clean the, throttle body, egr and Iac.