Diagnosic Assistance Requested -- Hesitation

[TomO]

VEHICLE: 1997 Mercury Grand Marquis, completely stock, mileage ~ 150K, great condition, well maintained.

BACKGROUND: Car used to be driven regularly, always performed well, then put into storage mode about a 1-1/2 years ago, started every two weeks while in storage, battery died last fall, so been jump starting every two weeks.

PROBLEM: Month or two ago noticed that if the throttle is opened slowly, everything is fine, but if the throttle is opened quickly there is a hesitation or stall, then the engine picks up and follows the throttle input. There is a clearly audible "PUFF!" sound that appears to be coming from inside the throttle body when the throttle is opened quickly. Installed new battery, but of course, that didn't solve the problem (starts a lot easier though!).

Other than this hesitation problem, the engine seems to be running perfectly (no miss or vibration or weird sounds). However, before I put the new battery in, I noticed that sometimes there was a type of puffing/rattling sound coming from the exhaust and the exhaust seemed to be hotter than normal and also once a faint rotten egg (sulfur) smell, which tells me that the engine was running really rich. Like I say, though, that went away with the new battery.

Any help would be greatly appreciated.

Also, would it be worth it to take the car to somebody with an engine analyzer to see if the system may be able to show some sort of diagnostic code?

I'm wondering if I may have damaged something from jump starting the car for so long -- maybe some sort of electronic sensor or valve or something

[88grandmarq]

could be as simple as bad gas

[TomO]

could be as simple as bad gas

Thanks for the input. I'm hoping it's just bad gas, but I filled the tank about a year ago and at that time I also added an entire bottle of Sta-Bil. I would think that would prevent the gas from going bad in just one year, but maybe not.

I guess the only way to tell is to siphon out the old gas and put in some fresh gas.

[gadget73]

yeah, change the gas and the fuel filter and see what happens. Actually just topping it up with some fresh fuel and running the tank out should be sufficient, then change the fuel filter when you burn the tank out.

[DsTrBd]

Ths cats have been sitting and not warming up completely ! Go underneath and bang with a hammer (easy) and loosen some carbon... Take her for a drive on the freeway about 20 miles. That is some of the egg... D

[TomO]

Well, I finally got the gas changed out and -- just as I suspected -- it's not the gas. The gas was just over a year old and I had put a whole container of Sta-Bil in it.

I also took it for a short drive around the storage yard and when driving it, you can't hear the 'puff' sound like you can when you're standing over the engine bay.

When taking off from a stop, the car stutters and hesitates like it's missing, then it catches and runs. When idling, it and you open the throttle suddenly, it acts like it cuts out for just a split second, then revs up.

So, there's more missing and hesitation when the motor is under a load (as in driving it as opposed to just idling). That makes me think it's a spark issue, but I'm not positive.

The aggravating thing is that this has just developed on its own over time. The car ran great a few months ago -- no work has been done to it at all.

I guess the next thing to do would be change the plugs and possibly the wires? I pulled a plug and it looked pretty worn.

[DriverSideImpact98]

I think it might be your catalytic converter. The cat is probably damaged/the ceramic is melted.

That happened on my old 89 F250 with a 460. A plug wire was slightly off, and it caused an unnoticeable misfire and an incomplete burn and melted my cat.
It idled fine, but when accelerating, it would hesitate and feel like it had no power, and it was really hot and smelled a little sulfury, and I heard a clunking sound in the exhaust. Took off the exhaust behind the cat and sure enough the ceramic was in a big ball in the bottom of the cat.

The ceramic would plug up against the outlet of the cat when you accelerated, the plugged exhaust caused the hesitation.

[TomO]

I think it might be your catalytic converter. The cat is probably damaged/the ceramic is melted.

That happened on my old 89 F250 with a 460. A plug wire was slightly off, and it caused an unnoticeable misfire and an incomplete burn and melted my cat.
It idled fine, but when accelerating, it would hesitate and feel like it had no power, and it was really hot and smelled a little sulfury, and I heard a clunking sound in the exhaust. Took off the exhaust behind the cat and sure enough the ceramic was in a big ball in the bottom of the cat.

The ceramic would plug up against the outlet of the cat when you accelerated, the plugged exhaust caused the hesitation.

Hey, thanks for the input. That sounds logical to me. Maybe my weak battery was causing a weak spark, which caused rich exhaust, which overheated the cat(s). As I mentioned above, I did notice the sulfur smell, the very hot exhaust (so hot you couldn't hold your hand up to the pipe outlet), and a sort of weird exhaust sound at idle.

That would also explain the gradual onset of the symptoms over time from just starting the car every couple of weeks.

I'll check that out -- thanks again for the advice.

[gadget73]

sounds like a dead accelerator pump to me. Look down the throat of the carb with the motor not running and open the throttle. You should see a steady stream of gas shoot down the neck of the carb as you open the throttle. If not, the pump in the carb is not pumping. The diaphragms deteriorate and stop working over time.

[86VickyLX]

sounds like a dead accelerator pump to me. Look down the throat of the carb with the motor not running and open the throttle. You should see a steady stream of gas shoot down the neck of the carb as you open the throttle. If not, the pump in the carb is not pumping. The diaphragms deteriorate and stop working over time.

What 1997 Grand Marquis has a carburetor?

My first bet is to look on the back of the intake manifold. There's a rubber T that likes to rot away and cause nasty vacuum leaks. Follow the PCV valve lines. One should lead to the rubber T.

I'm surprised that the check engine light hasn't come on, with it being OBD-II and all...

[TomO]

sounds like a dead accelerator pump to me. Look down the throat of the carb with the motor not running and open the throttle. You should see a steady stream of gas shoot down the neck of the carb as you open the throttle. If not, the pump in the carb is not pumping. The diaphragms deteriorate and stop working over time.

The symptoms are exactly what I would suspect with a bad accelerator pump -- hesitation and missing when you open the throttle suddenly. But, as 86Vicky says, this car has fuel injection.

I don't know what takes the place of the accelerator pump on the sequential injection system. The ECM must measure some sort of input like throttle position or manifold vacuum (MAP??) and then compensate by causing extra fuel to be injected.

As you suggest, this could be the problem but I have no idea what to look for or test.

[TomO]

What 1997 Grand Marquis has a carburetor?

My first bet is to look on the back of the intake manifold. There's a rubber T that likes to rot away and cause nasty vacuum leaks. Follow the PCV valve lines. One should lead to the rubber T.

I'm surprised that the check engine light hasn't come on, with it being OBD-II and all...

I went through the entire engine bay looking for potential vacuum leaks when I first noticed this problem, but I'll check the T on the back of the intake manifold -- I don't think I specifically checked that.

That's a good point about the check engine light. To be honest, I don't remember for sure if it's coming on or not. I'll have to make sure to check for that next time I go out there.

One thing that might be messing me up is that since I put the new battery in, I've been disconnecting the battery after I shut the car off so the battery won't discharge (since I only typically start the car every two weeks).

However, maybe that's part of the problem. Maybe I need to leave it hooked up and drive it around a few times so the computer will recalibrate itself (?????). I don't even know if that is realistic or possible -- it's just an uneducated guess on my part.

Anyhow, thanks very much for the input -- it's greatly appreciated. Please keep the suggestions coming if you think of anything else. Meanwhile, as I have the time, I'll gradually try things that you guys suggest in order to narrow it down.

[gadget73]

What 1997 Grand Marquis has a carburetor?

apparently ones that my brain has a total fart about. not quite sure how i totally missed the first line in this post but for some reason I was thinking he was someone entirely different who has a carb'd GMQ. This is what happens when I post after eating far too much and having a couple beers with it.



and yeah OBD2 does have a learning curve and it requires a number of drive cycles to set itself up properly. Might be as simple as needing to leave the battery connected so it doesn't lose it's mind. How old is the fuel filter and the gas? If it sits a lot, the gas may be going funky and it will clog the filter up.

[86VickyLX]

and yeah OBD2 does have a learning curve and it requires a number of drive cycles to set itself up properly. Might be as simple as needing to leave the battery connected so it doesn't lose it's mind. How old is the fuel filter and the gas? If it sits a lot, the gas may be going funky and it will clog the filter up.

If all is well, car should run as it would from factory. If you have dirty Throttle body, or vacuum leaks that the computer can compensate for (after relearn), this would cause problems for replacing a battery. This is why I bought a 9v battery tender. All you do is stick it in a 12v cigarette lighter (that's hot at all times) or the OBD-II connector. Now the only issue there is that you can't have the door open or anything. No lights or anything like that. Or the memory will be lost.

[1987cp]

apparently ones that my brain has a total fart about.

Thain did a Michael!!!