Any Suggestions?

[TomO]

I have a 1989 Grand Marquis that is completely stock -- it has a 302 w/ sequential injection, AOD transmission, 3:55 rear diff ratio. It currently has about 138,000 miles. It's in pretty good condition. I've owned it for about a year and a half.

Lately, I've noticed what seems to be an engine misfire although it might be something in the transmission.

The symptoms seem to occur only at low engine speeds, high load and when I open the throttle and specifically consist of intermittent "cough-like" power loss pulses (for lack of better terminology).

For example, this might happen at about 40 to 45 mph in overdrive when I want to accelerate hard on the freeway. Or it might happen in Direct at about 25 to 35 mph when I try to accelerate going up a real steep hill.

There are no sounds at all associated with the symptoms. Sometimes it almost feels like I'm going over a bump. In fact, when it first started happening, I thought I had gone over a bump, but there was no bump in the roadway.

Otherwise, the engine and transmission seem to perform flawlessly with plenty of power and smooth running at high rpms. Idle is perfect too. I pulled the cap and rotor and checked it. There was a bit of corrosion, which I cleaned up, but there was no apparent effect on the symptoms.

I'm not an experienced mechanic, but my intuition tells me that if it were something like bad plug wires, then the engine would miss at high rpms, but that doesn't happen.

Any thoughts? Thanks.

[jklnhd]

Sounds like my car. I just shift it myself and lay into the go button. Actually, after about 1000 miles of doing that, its far better behaved when I leave it alone than when I got it.

Anyway, looking around the boards here I've narrowed it down to a misfire and/or a vacuum leak. Did you check the plugs? See if they're all white & crappy lookin, also check the timing. I say that first cuz checking that stuff is free.

[TomO]

I haven't checked the plugs or timing yet. I have suspected that it may be one or both of those. I'm pretty certain that there are no vacuum leaks. I went through the car really good last summer and made sure I fixed all the vacuum leaks, although that's not to say that one may have started.

Would this be caused by timing that is too far advanced? I suspect that my timing may be advanced too far because sometimes there is a bit of engine knock right after a stiff upshift into direct or overdrive. But that may be normal -- I don't know. This is the first automatic I've owned after 30 years of driving.

[1990LTD]

I had the same stuff happening to my car...when you're accelerating through gears 1 and 2 it's probably fine, but if you're just cruising along not quite fast enough to bother shifting into OD, does it start hiccuping? Put your foot down and it's fine, right?

I did a tune up on mine about two days ago and it fixed it. New Motorcraft cap/rotor/plugs, Ford Racing plug wires. Didn't do the fuel filter because I forgot to pick one up (I'm s-m-r-t)

edit: I'll add that I thought it was the transmission as well, until I brought it to a transmission man who took it for a ride and told me about a minute in that it's not the trans but in fact an engine miss. Buy the new stuff and compare the caps and plugs etc side by side.

[gadget73]

Ignition problems tend to show up at high load, low rpm. If the wires are old and/or the plugs are old, I'd start by replacing them. The misted water test in a dark spot might also tell you something. Idle the engine with no light around, and mist some water on the plug wires. If everything lights up, the wires are shot.

Might also want to replace the fuel filter. It can cause wierd problems, and they're cheap enough.

[TomO]

I had the same stuff happening to my car...when you're accelerating through gears 1 and 2 it's probably fine, but if you're just cruising along not quite fast enough to bother shifting into OD, does it start hiccuping? Put your foot down and it's fine, right?

No, mine seems to be the exact opposite of yours. The engine runs great just about all the time, except when I push the accelerator down at a slow engine speed -- enough to where it's just shy of downshifting -- that's when I get one or two big hiccups*. Once the engine speed gets slightly higher, it runs like a top. The strange thing is that it doesn't seem to do it all the time -- only sometimes. Like I say, in O/D around 45mph give or take a few and in direct or Drive around 30mph (when going up a steep hill and I give it gas).

It has never happened at higher engine speeds -- only at really low engine speeds. Almost like the engine is momentarily stalling. That's why I thought perhaps it might be due to over-advanced timing. I don't have a tach in my car, but I'm guessing the engine speed is below 1500 rpm when this happens.

*(this is another weird thing -- most of the time when I've had bad ignition wires in other cars, when you stepped on the gas you would get a stuttering effect, like the engine was continuously misfiring, although it may be a somewhat random effect. This problem is somewhat different -- it's like one or two or three big "jerks" or "bumps" then when the engine speed builds up a bit it pulls fine.)

[TomO]

Ignition problems tend to show up at high load, low rpm. If the wires are old and/or the plugs are old, I'd start by replacing them. The misted water test in a dark spot might also tell you something. Idle the engine with no light around, and mist some water on the plug wires. If everything lights up, the wires are shot.

Might also want to replace the fuel filter. It can cause wierd problems, and they're cheap enough.

Thanks for the tips. I'll try that. It could very well be due to old plugs and wires. I haven't changed them, and I have no idea if the previous owner ever changed them.

[gadget73]

If they're grey, look for a date on them. If they're original equipment you'll find 1989 printed on them.

[TomO]

If they're grey, look for a date on them. If they're original equipment you'll find 1989 printed on them.

Yes, they are grey and the look to be original equipment, so it's very likely that they are the original factory installed ignition wires. I'll check for the date tomorrow when it's light outside.

I guess wires must be a hit or miss (no pun intended) proposition. My wife has a 2000 Focus w/ the ZTEC and it only has about 80,000 miles on it. A week or so ago it started missing really badly out-of-the-blue and after taking it in for diagnostics, we found out it was bad ignition wires.

On the other hand, I had an '86 Mustang GT that I purchased new and owned until about a year and a half ago when I got this Grand Marquis. I put over 300,000 miles on that GT and I know for a fact that the plug wires were never changed. It had the original factory plug wires on it for 300,000 miles and it never misfired. Maybe those were heavy duty wires or something, or maybe I was just lucky.

[gadget73]

The originals last a long time, but they do give up eventually. I only wish that the replacement parts were as good as the stuff they left the factory with. As for the Focus, its probably got a lot hotter of an ignition system than the 80s Ford stuff is.

[dgouge773]

My 86 GM would do the same thing and I thought my fuel pump was about gone. I cut the muffler and tailpipe off and that got rid of 90% of the problem. The fuel filter, tune up and tps took care of the other 10%. I always wondered why these cars had such stupid exhaust. 4 converters, 20lb muffler and a "pigtail" tailpipe. My 86 F150 had lightning quick throttle response running original plug wires at 152000, but it also only had some rusty pipes and a glasspack for exhaust..