'94 Grand Marquis - 4.6 - noticeable miss - no code

Particulars: 1994 Mercury Grand Marquis with a 4.6 litre engine, approximately 118,000 miles, I’ve put on about 15,000 over the past 18 months. The car and I live in Las Vegas, NV. For those past 18 months, the car has been extremely reliable. During that period, I have replaced the battery, brakes, front and rear disc; the fuel filter and the air filter. It gets regular oil changes every 3,000 miles or so. I do not know the heritage of spark plugs, plug wires or the rest of the electrical system.

Recently, I developed a fairly serious miss. There were no “check engine” indicators. I took it immediately to my mechanic whom I’ve had for ten years. When he put it on the computer, it came back with what he reported were “no codes”. He suggested that rather than throwing parts at the problem, I continue to drive it until I had a trouble light. He also suggested that in the mean time, I add a bottle of fuel injector cleaner to the tank, and a second treatment with the next fill up.

This treatment did provide noticeable improvement, right from the beginning. I have now gone through the second treatment and the second tank of gas.

The current condition is as follows:

Toward the end of the second treatment, I have begun to detect a slight odor of fuel upon start up which had never been there before, even during the extreme heat last Summer,

There is a slight miss upon driving off which discontinues while getting into second,

Throughout second gear, it performs smoothly,

Upon shifting into the drive gear, it begins to shudder slightly, and remains until I continue accelerating ? this happens pretty much between 35 and 45 mph,

Once it passes this threshold, it smoothes out again, and can be driven effectively at normal highway speeds (55-65), and also reacts well while driving in city traffic at a steady 45 mph,

I would start with sparkplugs and plug wires.
Also, if you use gas with too high of octane, you will have a gas smell with that engine/year. 87 or 89 should be good, but 91 or 93 will cause the smell.

Thanks … one way or another, that couldn’t hurt.

Don’t over-fill the gas tank. Once the pump nozzle shuts off, don’t force more fuel in. To force more fuel in, could force gasoline to the charcoal canister, over-saturating it.
Have your mechanic check the purge action of the charcoal canister.

This car does not have that design issue.

It doesn’t have what design issue? The parts store lists charcoal canister and canister purge solenoid for it.

Thanks for the reply. Fuel tank is never overfilled. Also, I use only 87 octane gas, usually Chevron. The recent faint odor of fuel upon start-up is not the problem in question. That began only after a couple of treatments with the fuel injector cleaner. The primary question is what may be causing the engine miss, especially when it’s reading ‘no code’ on the computer analyzer. One thought was a Mass Air situation, but that has been ruled out.

The tank has a big enough (non-vented chamber) air gap that even if it is filled till gas is flowing back out of the fuel fill, once the cap is put back on there is still enough room in the tank for the gas to expand without it flowing out into the canister.
Can you find a single example of a flooded canister on a 94 Grand Marquis?

The ODB-I system on this car doesn’t have a miss detector like OBD-II cars made after 1996. The closest codes are for coil circuit faults. So, your on your own trying to find it. Try spark plugs and wires. Inspect the plugs for signs of possible problems. Compare them to this page: http://www.autolite.com/pdf/PlugTips.pdf

Also, use a mechanic’s stethoscope to listen to each of the 8 fuel injectors. They should all make a similar clicking sound. The one or two that sound odd are the suspect ones. Especially if they correspond to spark plugs that look suspect.

Thanks. Great information. Now I have a place to start. The graphics on the link page will also be very helpful.

What brand of injector cleaner did you use? Some are better than others. Techron works pretty good but BG Formula 44 works BEST. Since you tries this and things improved, I would use a product the REALLY WORKS and give it another try. Add the cleaner when your tank is below 1/4 full for best results. Then try a full throttle acceleration from 0 to the speed limit a couple of times. I mean WOT to blow the cobs (and deposits) out. If there is a misfire, this should certainly cause the CEL to light up…Since the plugs and wires are unknowns, it’s probably time to change them reguardless…

I would agree with tardis that you should start with spark plugs and wires.
Some of the 4.6 engines were prone to plug wire misfiring which is helped along by aged spark plugs.
When a spark cannot jump a gap on an aged/misfiring plug it’s going to do its best to go somewhere else; and that’s usually means through the side of the spark plug boot.

My memory is fuzzy on this but it seems the plug wires have been through a number of redesigns over the years.

It’s also possible that you may have another issue. You state the car has a shudder “shifting into the drive gear”. This could be a transmission torque converter shudder, which can be extremely prevalent when shifting into Overdrive. These transmissions are finicky about fluid changes (which should be done every 30k miles) and if the fluid has never been changed, now’s a good time to start.

Thanks to everyone who took time out to reply … replaced the plugs and wires and the old '94 Merc runs like Secretariat.

Thanks to you for posting back and letting us know the problem is solved.
The 4.6 engine/transmission is a good setup and will go a long time.
Mine has about 245kish miles and still runs like new while not even using any noticeable amount of oil between changes.

DO change the trans fluid. It is supposed to be done at least every 30K. Have someone drop the pan and change the filter and drain the torque converter by its drain plug. Don’t let anyone tell you it does not have a drain plug unless they check. If the TC is original, it does. DO make sure they use MERCON V even though the original fluid was MERCON. MERCON V is better for this application because it forestalls the TC shudder that will develop with MERCON. You will need 13-15 quarts.

A great place to learn about your Merc is www.crownvic.net

The appearance of the spark plugs can be used as a diagnostic to determine what is going on inside your engine. Here is the best “show and tell” on reading spark plugs which I’ve found: http://www.autolite.com/pdf/PlugTips.pdf