Hello,1978 Merc Grand Marquis.351M,5.8, Starts great,runs great cold,after warm up,vacuum fluctuates very erratic,from 13 to 18 and so on,maybe lower,but at least down to 13 or 14 and goes up to 18,17,20,and back down,fast changing not slow,,and eating fuel.This is what has been done.
1. Timing chain,,65,000 miles,was due,loose,as I did leaking water pump and new thermostat.
2.Replaced the Intake Gasket,Turkey pan,seals the end seal corners with ATV,as before front spraying Carb cleaner at front right of manifold would kill motor,thought I found the problem,or one problem.
3. Rebuilt Carburetor,new Base Gasket,cleaned out EGR plate,aluminum plate but not corroded,and left old Power valve in Carb,but cleaned it,new one looked different and thought this is a stage 2 power valve??.Set float to 3/8 if I read the specs correctly,and idle screw to 2 1/2 turns out as where they were before.Specs hard to understand,either 3/8 Th's or 7/16 Th's,,ANY HELP WITH THIS WOULD BE APPRECIATED IF I AM WRONG WITH THE SETTINGS?
4. Replaced Vapor Canister and Purge Valve as the valve was melted and the canisters bottom broke off,plastic and mounted close to the road on a bracket.
5.Check Vacuum from Trans Modulator Valve to Vacuum tree valve on fire wall,OK.
6.Check vacuum lines and cannot see or hear anything,but the Black Coffee Can on left fender,epoxied a few rust holes,vacuum test OK now.
7.Taped off EGR valve to intake manifold,no change at all,hope correct vacuum line going to valve,,it is off a vacuum T that goes into the thermostat housing,like a Temp sensor valve.
8.Sprayed Carb and Brake cleaner all over,even Carburetor base,all vacuum lines I could see and reach,no difference in vacuum on gauge or engine erratic idle.
9.Set timing the 14 degrees as per sticker on valve cover,and emission sticker on hood is gone.
10.Put premium gas in the tank thinking bad gas,no change.
11.No change when messing with Idle Mixture screws,from what I can tell,set them to 1 1/2 turns out,no difference compared to 2 1/2 turns out.
Always restarts OK,seems to hold and not die in gear,but out of gear,sitting,this is what it does.
Has anyone had this issue,I know the trans leaks a bit out of the bell housing,cannot see this causing a issue??
Where should I go now,is it possible that the Carburetor may have a internal issue,seems like a vacuum leak,but I cannot find anything,PCV valve seems to be OK,cleaned it and vacuum lines OK to Carb. Possible WEAK VALVE SPRINGS?????This car is in beautiful shape and been garage kept for many years as this was my late Mother's baby,and my Father does not drive is as it has sentimental values,and would like to keep it running great.

Any ideas would be APPRECIATED,a little rusty on Carbureted cars,trying my best,but can anyone throw a few ideas at me,my father is starting to doubt me,but I would like to make him and keep him happy with Mom's car.Thanks to all who respond,PLEASE HELP ME

 

First- I don't know what carb you have.
Next- did you get an instruction sheet with the carb rebuild kit, you should have~.
The instruction sheets generally have the specs for the carb based on the mfgr and carb model.
The only way to rebuild a carb is the right way and that is according to the instructions in the kit. You MUST do each step in the sequence provided in the instruction sheet(s). When those instructions are followed and the carb installed, it will start and run,,, then you do the fine tuning, setting the choke,fast idle speed, and when warmed up and the choke is fully open , then you adjust the idle speed and mixture according to the instructions..When everything is completed all you should need to do with a cold start is depress the accelerator half way to allow the choke to set and the engine should start and the choke pull off should open just enough to provide a smooth running engine if adjusted per the instructions. How carefully you follow the instructions will determine how well it will start and run cold and warm.
It is a great feeling when you get everything right and it runs like a clock. That is the only way I have found to do a rebuild aside from soaking and cleaning thoroughly everything except the choke pull off . Soaking that can destroy the internal diaphram.! A lot of guys figure if it ran OK before you don't need to go through the steps, but things change and a good rebuild takes all that into consideration , there are no short cuts if you want a professional job. If you have compressed air to blow out the passages that is part of the job.

 

Hello,thank you for the reply,everything is cleaned out of the carb as it was soaked over night in carb cleaner. I noticed yesterday messing with the carb again as I took it off,I noticed that there is a metal plate of some sort that was rusted or in bedded in the lower manifold,( the thick gasket ) going to the engine manifold side. I was not aware of this when installing the new gasket,and am wondering if this may be a problem,and that I noticed I may also not have one of the thin gaskets installed either on the top side of the thick gasket.If I am correct it goes like this from manifold up
1.Metal Plate
2.Thick Gasket
3.Thin Gasket
4.EGR Aluminum Plate
5.Thin Gasket,,,,( Second One Used )
6.Carburetor
If this is the way this goes,this may be my vacuum leakage,and I did notice some fuel leaking from behind the carb lightly streaming down the EGR plate,from the carb base gasket,so I am picking up the Metal Plate today as it was ordered and keeping my fingers crossed.I do not mess around much with carbureted cars,mostly I am familiar with the newer cars,and I worked for Chrysler,Plymouth,Jeep for 17 years until the back gave out,and I started in the business when the Fuel Injection started and they gave the carbuerated cars to the older Techs who new these carbureted cars,and can work on them blind folded.I will keep you up to date soon when I get to put the plate on the manifold and button her up.Thank you for your advice.I set the float to 3/8 Th's,as the data sheet they gave me did not have the 1978 carb settings for that motor,a bit weird with this spec sheet,but found the settings online..The carb number is,well a Motorcraft D8AE-TA,,,and some other numbers on the tag are H 8E 1,,,no idea.Thanks again,and will update you all,Mark Bradley...Hopeful,,,,

 

Hmmm, I remember the 351m as having the thin gasket under the EGR/spacer and the thicker carb base gasket between plate and carb only. Don't recall a metal plate? Don't recall an EGR cooler..

The aluminium spacers used to corrode and leak or bleed vacuum, warp (more so if thick gaskets used and overtightened).
If you suspect a vacuum leak, an old trick was to use a shop rag and 'gently' introduce it into/over the intake air flow as you slowly start to restrict the intake air. If there's a vacuum leak, generally the engine will smooth as long as the airflow isn't over restricted. Same goes with using the choke plate, but sometimes the fast idle cam gets in the way and an desired idle cant be maintained.

Any pics available do we can see under the hood. ;-)

 

Hello,to all,I figure the problem out. There is also a thicker composite gasket that goes under the thicker gasket,the larger mounting gaskets.So from the intake to the carb,
it is called from Orielly's,a Spacer To Manifold,,then the thick gasket,then a thinner gasket,then the aluminum EGR plate,then another thinner gasket,then the carb,I did not have the lower Spacer to Manifold gasket installed under the thick mounting gasket. On the old thick gasket,there is like a thin metal gasket that was in-bedded into it from years of being on the car,and I did not see it until I took the carb off again and was poking around and noticed this thin metal gasket in-bedded into the thick gasket.I went to Orielly's and sure enough,there was one more gasket under the thick gasket,and I did not have this on the car.I installed it yesterday,and WTH,it ran like a brand new car,blew me away as I did not know about one more manifold gasket.Been 30 or more years since I played with a carbureted vehicle,and was not aware of one more gasket to be installed under the thicker base gasket,totally embarrassed about this,feel like a real dummy now.So,to all who helped me,you were all giving me great helping advice,but this was totally my fault,and no where is there a picture of what goes under the Motorcraft 2150 carb with the EGR plate on a 351M and or the 5.8 motor,looked every where,but it showed up on Orielly's parts page,but no picture to show the procedure of the gasket placement. So if you come across this,look on the bottom of the thick gasket that goes to the manifold,and you should see this,as it was a thin metal,steel gasket and you should be able to crack or peel it in pieces with a small screwdriver,and this will let you know there is one more thin gasket.It is composite material now,it should not rust and deteriorate.So to all,THANK YOU FOR YOUR HELP AND ADVICE,great forum and great people to help out.Good luck to all on your projects,I think I will take a break on cars for a while,this has been going on for about 4 weeks,now I am behind on raking.Thanks again,Mark,Be safe,and enjoy your wrenching,as I love it,Mark,,,PEACE and Happy Holidays to all.............................................

 

Don't know if you might come back and read this, but thanks for sharing the info with us and glad you solved the problem!

 

No problem hanky,always glad to let all know any issues to help out for another with any issues,the only problem I have now is when I got accelerate,the AC go hot,I must have a vacuum hose backwards. But I cannot find a vacuum diagram for a 1978 Mercury Grand Marquis,351M,,5.8. I know that I forgot the mark the vacuum lines near the rear intake manifold port,one goes the the vacuum T,but I have at least 2 lines,one with a T and vacuum valve,and one to a large coffee can reservoir,and there is another smaller reservoir with a vacuum line going to it,but I have something screwed up,and I know the EGR line comes from the thermo vacuum T up by the thermostat housing area.The only schematic of the vacuum lines I found is from Autozone,but it is a schematic of a Canadian routing,so going to hunt around,and no sticker on the car.Hope all is well,thanks for the reply and help my friend,Happy Holiday's,Mark and Family...

 

Try unplugging the MAF sensor while the car is running and if the idle changes at all then either replace or PROPERLY clean the maf sensor. if this year doesn't have then what does it have that is simular to maf?