Good day to all,
Well through numerous hours of troubleshooting (86 F150 4.9L L6), Carb replacement, complete tune up, and timing check, I believe there is a intake leak. The previous owner said he had a local guy replace the engine with a rebuilt and had to have the In/Exh. gasket changed once already. Well I hook up the vacuum gage to to Map Vac. line and low and behold it is jumping fromabout 10Hg to 17Hg. According to what I know this indicates Intake leak, I can pull 19Hg at RPM, however any time spent off the pedal it will duplicate the same symptom. Compression check and plug inspection showed #6 took way to many cranks to build up to 130 and the plug was a little black all other plugs burning good (Motorcraft) So I am thinking the intake is leaking in that area, all other cylinders popped fine between 145 to 160. Soooo, anything for me to know before attemping this. I have done this combo manifold before on a slant six (Mopar) during a rebuild. Just thought I would see if there are things to look out for. Tips from others with experience on this application are a very valuable tool when doing any kind of maintenance. Well Thanks for all the Help
Joe

 

Joe,

An intake leak will not affect your compression test, which ideally is done with the throttle propped open. I recommend you do more trouble-shooting before doing anything to the manifolds, other than a re-torquing in place. (You can squirt oil around the area of the suspected leak. This is messy, but does give you some positive indication of the location of a leak.) Another data point is that a vacuum leak in the #6 area would make #6 run lean and you observed a black plug which indicates a rich or mis-firing condition.

A leak-down test will allow you to find out more about #6 cylinder. If you need more info, just do an FTE search on "leak-down". I would expect a valve or ring problem - sorry if this is bad news.

 

check the kind of gasket you get, as there are different kinds. if you have the stock intake and exhaust manifolds, its an intake only gasket, if you get the double gasket then you will have another leak. the exhaust manifold sticks out a little farther, so if you put a double gasket on the head, the exhaust will keep you from getting a good seal on the intake. look for an actual intake gasket, they can be had, you just have to make sure and get the right one...

 

Acheda,
You have a point there, however, the book I am using to assist in the T/S process tells me if the vacuum gage fluctuates between 10 and 20 then its either a carb misadjustment or intake leak, it is a bit confusing. I can only get the truck up to 72 MPH on the higway, it runs great at that speed and will run that fast all day long with no problems, it will idle a bit rough but it will not stall. I intially thought it was a miss so I took what I believe to be the corrective action. Seems like no matter what I do the symptoms will not clear, the valve and or the rings came to mind when taking the compression, but, it did build up pressure, does teh amount of cranking required have anything to do with it. I have no problem tearing into the engine, I used to inspect Very large diesel engines for the Navy once upon a time. I am not the swiftest with the hole carburation thing!. Navymidn08 did I understand correctly about the single gasket only? I do remember reading some thing about use only graphite greae for the exhaust manifold, I cant remember if it was alldata or the chiltons. Anyway I will search for the leakdown testing, and I will dig up the procedure for the gasket replacement. I do believe the manifolds are the stock manifolds.
Joe

 

Thats not an indication of a intake leak. If it wanders at idle and stabilizes at higher rpm, its a sign of a burned valve or some other leak in the combustion chamber. Its most likely a valve just by the gauge readings, but with the plug being oil fouled too it could be a ring or a cracked piston.

 

Oh, how many of us have been where you are, watching that needle struggle up. It does not help much psychologically if it is the last cylinder after the rest are high. Try a consistent rule: Mine is to go three more jumps after the first "good" one. Then you can compare the numbers. A completely dead cylinder can just be zero, but a flake of carbon under a valve, a cracked valve or seat, a broken ring or burned piston problem can all have the compression gage symptoms you are describing in their early stages.

Believe me, I am not wishing problems on you. All I am saying is that a leaky intake will have zero effect on a compression test. You should get the exact same numbers if you do a compression test (with the throttle proped open - don't forget to put it back when done with the test) AND if you do a compression test with the manifold OFF completely.

Here is a link about leak-down testing (read it all to get the whole idea):

https://www.ford-trucks.com/forums/5...ml#post4513368

I'll be happy to answer any questions you might have.

 

Acheda,
Well That was an excellent link!!! I think before I go yanking the manifold off I will set up and do another compression test. When I did it last I only did the Low cylinder(#6) twice. I did not have the throttle open all the way, I know it might not have a great difference, however, I am one who believes in doing things the right way all the time. So, I will set up and do it again, I do find it interesting that the cylinder did build up to 130 psi and hold it untill I released the pressure. I am sure there is some kind of anomolly happening in the #6 cylinder, this engine was a rebuilt that had been put in about 3-5 thousand miles ago. I dont know the builder and didnt see it go together so that doesnt help much. I will also try the spoonfull of oil in the cylinder if I cant get a good reading the first time, I will focus on that cylinder sice the others tested high, according to the High/low compression chart as long as a reading is withen I believe its 60% of the highest psi it is good, and since 160 psi was High and 130 low i am safe, however, it is the amount of time to build up to 130. I remember doing it and the first 4 cranks produced about 75 psi, so I stopped, releasedf the psi and did it again, the second time I let it crank about 8-10 times before it popped to 130 and held. Well I will continue to T/S. Thank you so much for the supportive information. I'll let you know how it goes
Joe

 

Your compression tester has a one-way valve to hold the gage at pressure. This does not mean that the cylinder is holding the pressure. This is what the leak-down test does.

The oil will help separate ring problems from bearing problems. The rules of thumb on 60% (or 25 psi max spread) are a generalization. This is not a situation where if you are in tolerance, you are good. If you are near the tolerance, you need to keep an eye on it. Carbon build-up can make a cylinder high; carbon flakes can (temporarily) make one cylinder low. If you get the same pattern a week later, you probably have a problem that you will need to deal with sooner or later.

 

Thank you for reminding me, I forgot all about that. You are correct. Since it has been a little more then a week since the original readings the next set should be a good comparison, worst case I just hope its some thing to do with the head. But I have every thing needed if its a little more deeper into the engine, its real a fine truck, just need to get to the bottom of this drivability issue or lack of!!
joe

 

Okay, I have the leak down test kit and have air, I will set up to do the check tomorrow 4/18. So, assuming #1 TDC rotor pointing at the #1 plug, valves closed, I should only have to rotate the engine till the rotor points at the #6 plug post then bring #6 to TDC being #6 seems to be the problem cylinder. Pump the air in check and listen for air through the dip stick tube and the carb. throat, however, I would think a rapid loss of air would be a valve problem and a slow loss would be rings. Rapid loss of air check for sound in intake manifold and exhaust, this should point o which valve is bad. Air frim dip stick tube would indicate ring problem. Well I hope I got it right.

Joe

 

Sounds like you understand what to do.

On more tip. You can pressurize the cylinder at TDC, but if you do not lock the engine, it may suddenly go to BDC (in either direction). Make sure your hands are out of the way of the fan & belts. Put a manual trans in top gear with the parking brake set firmly & the tires chocked. (Auto, see below.)

Being that the valves are the most likely problem, I'd suggest setting the engine past TDC in neutral and letting the air push it to the bottom of the compression stroke where both valves will still be closed. This will be the easiest and will do just fine for testing valves. You also might want to do at least one other cylinder for a comparison.

If valves are not the problem, you can see what the rings do at TDC and BDC. I never tried that as I was always looking for (& found) bad valves. Good luck!

 

Well, the leak down test is complete. Didnt realy help much, Can hear hissing from the valve cover, the again, I did #6 and #1 with the same results, Engine was cold so Maybe I should have done it hot, either way I will be pulling the head. I re did the compression on both cylinders witht he throttle wide open, well the #1 pops 160 psi while the # 6 wont pop more the 50 psi, I did the test on #6 with a bit of gear oil in the cylinder and still didnt get more then 50 psi, so at a minimum I need to pull the head and get a look see, I think with the oil in the cylinder if the problems were ring related the pressure would have gone up, but no joy in whoville for now, I will start tearing the head apart tomorrow, there is a shop in town that will rebuild the head for a decent price, I just hope to see some thing deffinate when I pull it off.
Joe

 

Joe,

I know you are not happy, but it is easier (& cheaper) to only have a problem in the head than in the block. Cold vs. hot can be an issue, but a low cylinder is a low cylinder and will only get worse. I had a VW that consistently would have an even compression test cold. It still seemed to be missing on one cylinder, which is pretty noticible on a 4-cylinder, so I finally did a hot compression test. Cold test was 125 for all 4 cylinders - hot was same except #3 dropped to 75. This was a valve that would take on a slight warp when hot, but something you want to catch early in an air-cooled engine.

Hissing in the valve cover? None in the exhaust or intake? Did you look for bubbles in the coolant. Not much comes through a closed thermostat, so this is a time for a hot test.

Another thing to do is to carefully inspect the head gasket's "print", right after you pull the head. The steel ring around the chamber hole in the head gasket will leave a print in the carbon around the cylinder. If you have a combustion gas to coolant leak, this can often be detected by a careful inspection of the gasket and the marks left on the head & deck surfaces. Of course, a blown gasket is very obvious, but a leaking one can be detected by a careful inspection. Just look for a small zone that has a different appearence than the rest of the seal areas.

Another thing to inspect is to stand back and look at the color patterns of the valves. Intakes and exhausts will be different, but focus mainly on the exhausts. A leaky valve will have a different appearance. It may even have an edge that is grossly different, showing where it was leaking from a warp or crack. Good luck & let us know what you find.

P.S.: ONE MORE THING. Did I understand that the leak-down rate was the same for #1 & #6? If this is so, you might not have an head problem, so do not pull the head until you look a little further. One thing that could cause a poor compression test, but still have a good leak-down is a valve actuation problem. (The valve opening affects the compression test, but the valves are closed during a leak-down test.) Take a good look at the rockers while the engine is cranking and make sure that both valves in #6 are opening properly & fully. There may be something I have not thought of, but do a little more trouble-shooting before tearing it apart.

 

What he said. I suspect you may have a slightly bent valve or a tight guide. A burned valve wouldn't pass a leakdown test. Sometimes a bent valve will close, it just takes it a little bit to close completely and that allows the compression to leak back into the intake.

 

I'll do a "what he said" as well. One short-coming of the leak-down test is that it is static. I now see that if a sticky valve happens to close most of the way, the pressure in the chamber from the leak-down test will seat it. Ditto for a slightly bent valve. I am learning something from this thread.