Engine stumbling at 30 and 60mph

[dehlatem]

I'm going to try to be as detailed as possible here. Sorry in advance for the length.

This is an 85 grand marquis with 135k on it (new plugs/wires/dist cap and rotor, coolant flushed, oil just changed, trans fluid looks good).

Starting from a standstill on an open road, I can get to 30 fine with no stalling, but maintaining 30 is when things get rough. Slowing down to 25 and re-accelerating to 30 is also very rough. I'm light on the throttle since this is an older car, but if I floor it past the 25-30 mark I never notice any missing/hesitation. I can't tell if it's shifting after that speed or not, but it doesn't get rough again until 55-60.

If I'm going under 50 I keep it in drive, on the highway I flip it up into OD.

The local garage told me it could be a bad cylinder, and the engine would need to be replaced if that were the case. The transmission went out in my daily driver, so this has become my daily.

Any other ideas on what could be going on?

[Tiggie]

I can say pretty confidently that that shop is trying to take you for a ride. There is no way you'd need a completely new engine with the problems you are describing.

These cars get lots of vacuum leaks, I'd check for those first. Does it idle nicely?

So the stumbling clears up if you floor it? That would seem to eliminate a fuel delivery issue (read clogged fuel filter).

1985 has EEC-IV. You can check the codes with a code reader or by yourself. Check out http://www.fordfuelinjection.com/?p=13, but be advised you can't use the flashing check engine light method since you don't have one.

[gadget73]

sounds like a vacuum leak or a braindead EGR system. Mine was doing this when the egr regulator was acting up, the valve would go to 100% open and it ran like crap between any speed over idle and about 60. flooring it was fine. See if any of your vacuum lines are cracked, and you might want to see about cleaning the carbon out of the egr plate, if there is any in there.

[dehlatem]

Good info.

It idles perfectly now actually (after tuning it up - I replaced the fuel filter at that time also). Almost makes me nervous with how quiet and smooth it purrs (I'll think the engine has died because I can't feel anything).

I guess I figured if there were any codes I'd see a check engine light telling me it threw some numbers out. I'll see about getting that read.

And is vacuum something I can check/diagnose relatively easily? I got an EVTM book off ebay for my model/year, but everything inside is difficult to decipher. I'd love to spend a weekend tearing out old vac lines and replacing them, but I'm afraid of making things worse. I asked the garage to look into this for me, though I'm not confident they really even checked anything before diagnosing it as a cylinder problem (which would have cost me $100 just to check compression).

Another thing to add - When I changed the plugs, all the old ones seemed to be in pretty good shape, just really old. It didn't appear that they were oil soaked or were covered in carbon. PCV valve changed recently too.

This has me scratching my head for sure.

--edit

Thanks for the reminder gadget. EGR was mentioned in my last thread, but I didn't want to necromance it again. Forgot all about it.

[gadget73]

no check engine light on these cars before like 1989. if its running good, leave it alone lol.


the vacuum lines are mostly a visual inspection. if any are cracked or the rubber looks like its ready to split, replace the line or patch if its plastic. if you have to replace lines, just do them one at a time and you should be good. Actually most of the engine's vacuum lines are not in the EVTM anyway. You'd need the emissions manual for that. The EVTM has the vacuum for stuff like the cruise and climate controls.