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FORDFXR, Ford Mechanic
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I have a 1986 f150 with 4.9L inline six; I have been having

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I have a 1986 f150 with 4.9L inline six; I have been having problems with "rough Idle" on the truck; To date (poking around) I have replace the Carburator (helps a little, but does not solve issue), replaced the plugs, rotor, cap and vaccum line to EGR valve (note; when I manually move the diaphram on EGR, the engine lugs). I have adjusted the timing (at least I think correctly) to 10 BTDC, with spout disconnected to get base timing. But do want to note that I don't get proper timing on #1 cylinder, but do get it on #2 (with induction light), which is very strange....and there was only a single timing mark (wide notch) on vibration damper. Any ideas here?


how badly worn were the plugs you took out?

what brand of plugs did you put in?

what shape are the wires in?

if you have a close look at the damper you should see marks on it, 2 degree graduations, if it were marked with paint, it was by a tech, they do not come that way.

if your inductive light will not pickup on #1, use #6 it is the running mate (it fires at the same spot on the crank, just 1 revolution off.) -- if the light would not pickup on 1 there may be a concern with that wire/plug.


Customer: replied 8 years ago.

1) Had Autolites (dual Platnums) in, and did like; took them out and they were "white and clean" (100K on engine)

2) Put in "Motorcraft - plain janes (nothing special, in as some engines don't like the platnums)

3) I replaced the wire at 65K, and they (at least by looks; haven't measured resistance) are not cracked, or too worse for wear. When I remove each one individually, I do notice a change in the engine "roughness", not that is always a fail safe test.

4) The only markings I noted on the damper is a red highlighted notch (someone who knew what they were doing marked it). I did not remove spark plug #1 to check top dead center, but this was the only marking I noticed when quickly hitting the starter, and cheking closely for other markings on the damper pulley. This is what is confusing me; I do get it to line up of #2....and that is it.

5) didn't think to use #6, but the light is firing when connected to #1, just don't see any timing mark for reference on the Damper.

i always recommend that you install the plugs in an engine that the manufacturer put in ie; ford gets motorcraft, gm gets ac/delco, dodge gets champion Japanese gets ngk etc... i have found that any other seem to give you grief after about 20,000 km, and this always seems to lead guys down the garden path because "the plugs are only 6 months old they have given no problems -- how could they be bad??"

on the damper i will take a wire brush and clean it while the engine is running (WATCH YOUR FINGERS, YOU WILL NEED THEM TO PLAY THE PIANO!!!) then have a close look at the damper, there will be marks there, and mark 10 degrees with a white paint stick

if the light will not fire on 1, use 6 it is the running mate (firing order is 1,5,3,6,2,4)

by the way, have yo checked for vacuum leaks at the intake manifold gasket?

Customer: replied 8 years ago.

I hear you on the plugs...the exact reason that I went to the Motorcraft for troubleshooting (I don't think the older engines were designed for Platinum plugs..but anyway).

I did in fact clean the damper with engine degreaser, wiped in down as "clean as a whistle", then got under the truck and "spit cranked" with remote starter, and looked for other markings (other than the wide red-marked indication), and saw "Nothing", which blew my mind...yet I digress. So the botXXXXX XXXXXne is that, I would think that I would have found another (perhaps thin marking) score mark, but found nothing, even outbourd near the two belt pulleys!! Short of making my own reference for TDC on #1, I'm not sure where to go with this (my concern in doing this, is it is not very accurate). But the real confusin thing is, when I go to #2, I do get a reading with the "red" marked wide notch, close to where the engine wants to run pretty good (but seems to "Miss"). I have tried to time it by ear, and it only levels out when the timing ( with light on #2, using the red marked wide notch) is beyond 16 deg. BTDC. Strange.

I did spray carb cleaner around the carburator, and carburator base and TPS assembly, and did not note a change in the engine performance. I did not check around the Intake manifold (was afraid of the results!) as of yet...but it is a thought. What is it that is making you think of vaccum as the source (over say, electrical) BTW, I did replace the coil and rewired the condenser on the coil. No difference.

What is really bothering me is that until I can verify base timing, the rest is a "crap shoot"! Ideas (procedure) on setting this, given I have not been able to isolate the correct marking on the damper (I don't think a mark exists on it, spent over an hour looking for it, thinking I missed it each time! pissed me off to no end!).

i think i owe you an apology - there are timing marks on the timing gear cover-- therefore there will be only one mark on the pulley or damper

i have looked in all the books i have here, i cannot find a pic of the pulley and timing marks anywhere.

still looking


Customer: replied 8 years ago.
That's O.K. - just so you know, on this particular engine ('86) there are timing references on both the dirver's side (on time gear cover) and also on the passenger side (with markings from 6 deg ATDC to 16 deg BTDC), with a "sighting hole" off of the 2 degree marking increments, just to add to the confusion. I have been using the passenger side markers (while also checking the timing gear cover markers to see if I can discover anything). Also tried the timing gear cover markings using cylinder #1, but with no success either.

if you want to figure out which marks are the ones to use pull #1 plug and bring the piston up, rock it bake and forth at tdc, then have a look at the marks, this will tell you which ones to use

i was just looking at your first post, you mentioned that you moved the diaphragm on the egr valve and the engine lugs, i have seen egr valves leaking, try pulling the valve and making a gasket out of cardboard (from a cereal box) with no hole for the gas, this will block the egr and if it runs well then you know that there was a leak from the valve.

Customer: replied 8 years ago.

O.K - I have once tried this with a long screwdriver (to find the top of the stroke of the piston on #1), and needless to say, got me in trouble. Two questions come to mind;

1) what is a good method (instrument to use) to determine I am at the top of the stroke on #1, without causing damage (I will be working alone)?

2) and, just in the event that wires are crossed, is there a reference on the distributor cap (the one I have does not have markings for the cylinders) that I can use to make sure that the rotor is addressing the correct cylinder?

Your idea about the EGR gasket is a good one. But at idle, should the EGR have the affect that I noted earlier (I thought that the EGR should only be active during driving speeds/rpm, is this in fact true? and if it does affect the engine performance at idle, something is wrong with the EGR (or vaccum lines to the EGR)?


egr for that year should be off at idle, if the valve is leaking, it may give you a rough idle

if you pop the dist cap off and mark where #1 is then turn the engine till you are close, remove the plug and look in the hole you should be able to turn the engine by hand, and watch the piston come up - when you get it close to the top, you should be able to see how close the mark is to the timing marks -- you just want to know which marks to use

generally #1 is marked on ford dist caps


just so you know, i have been getting a little confused, i have 4 customers on line who all have f-150 sorry if i seem a little scattered -- yes i get that way!!

Customer: replied 8 years ago.
O.K. Thanks- unfortunately, this is an after-market cap, and not marked. But I will for now assume that the distributor set-up is correct (otherwise I am sure it would backfire, and not just miss!). I will try the EGR, which may have a small leak (but when I lightly depressed it to activate it, the engine did lug). I will also check the intake manifold for vaccum leaks (I hope not!) and go from there. So being I have never done this before (i.e. accepting your advise), where do we go from here (want to assure you get credit for your time). BTW; I can understand why you have 4 customers with 4.9L issues. When the are set up correctly, they are great. When they are not running correctly, they are tempermental, and need to "stroke" them (no pun intended). Thanks for the help.

last night i had about 80 questions open,

the 4.9l in my opinion is one of the best engines ever built, we had one on the farm when i was growing up - my brother and i could not kill it, i have owned one and loved it - not necessarily powerful but strong maintenance wise.

your engine is running so the timing is close, follow #1 wire to the tower it is on and mark the dist just below the cap with a sharpie, this will give you a close idea as to where #1 is

lets keep the q open until we have it fixed


Customer: replied 8 years ago.
O.K. - your a dedicated man! Let's call it a night, and I will follow your guidance tomorrow morning. Will let you know how WE made out tomorrow - Thanks for you patience and DILLIGENCE on this. Have a great night, and thanks for your support!!



Customer: replied 8 years ago.

Mr. FordFxr; I am the guy from last night with the 1986 f-150 4.9L 6 cyl, with the "Miss"-

Here is what I learned in school today; First, I took out the plug in cylinder #1, and hand-cranked to engine to find TDC, after playing with it a while, I got to what I thought was TDC (turned out later to find it was about 2 degrees off). I then carefully inspected the Damper and pulley for timing marks, and were NONE, other than the large notch (which when at TDC on #1, was located directly down at the bottom of the engine; near oil pan). I then took out a hack saw blade, and made my own small mark at 0 deg. TDC, and marked it with paint. After replacing the plug, I disconnected the spout, and started the engine. With the timing light on #1; my mark was seen at about 2 deg BTDC (2 degrees off). When I moved the light to cylinder #2; The "red" wide notch was exactly at 0 deg TDC. When I reconnected the spout connector, the shifted to 12 BTDC on cylinder #1, then when I checked #2 (with "red" wide notch on damper) it was exactly at 10 deg BTDC. So, with it all, it looks like the timing is good (although I still do not understand why the factory timing mark is that far off, with no secondary markings aligning with either the passenger or driver side timing guages??).

However, the truck was still periodically "missing".

Next, I took some carburetor cleaner and went over intake manifold (at the head), and noted no difference in the engine idle. I did the same across the base of the carb. and TPS assembly, and had a very slight increase in idle. So, I tightened the carb a little bit (but not much, in as it was really not loose), and repeated again, with no change in idle speed. Also sprayed all of vacuum lines, and they seemed O.K.

Next, I took you suggestion from last night and removed the EGR valve. Upon inspection; this was the "nastiest thing" that I have seen in quit a while. It was horribly carbonized, and crusted up all over (on the rear diaphragm section). I cleaned it up and put the cardboard gasket in (Using a little RTV on either side of the cardboard), reinstalled it, and then started the engine, and it did in fact make a difference, but did not take care of the entire "miss", but it did help. Found out from Ford that this part is obsolete, but was able to have Ford locate one, which will be here Wdnesday (showed the parts guy at Ford, and he indicated "it was definitely shot!'). Now, I continued to investigate (with EGR Blocked off), going back to the electrical system; and noticed that when I pull the plug wires on #3 or #4 cylinders, it had NO affect on the idle. So I changed those two plugs, with the same results. Next, changed in a new set of plug wires (just in case), and again no better (in fact, I went back to the old wires). Now the truck is running "crappier" than it was originally. So now I am back to the original plugs and wires, and it is still running "nasty". The miss is now persistent, and not occasional as it was before. Could blocking off the EGR have something to do with this? I remain confused with what I am seeing.

cool B+ on the home work,

to make a cylinder fire you need compression, fuel and spark at the right time

with the egr blocked off the engine will not run rougher-- if the valve was leaking, blocking the egr will make it run smoother.

try advancing the timing to 10 degrees

check for spark at cyl 3 and 4 (just pull the wire and install a plug, make sure the base of the plug is grounded, start the engine - the spark should be a nice blue spark)

do you have a compression tester?

would like to see the results of a compression test - how many miles on this engine?


Customer: replied 8 years ago.

Sorry - didn't get your message until late (I am Eastern Standard Time; your 3 hours earlier than me, in Alberta).

I had similar thoughts on the cylinders; I do have a compression tester and will next map out the cylinder compression (but it had run a bit smoother earlier...very confusing) on each cylinder to get an overall balance of compression. Will check spark as well on all cylinders (am getting on each into plug; pulled each one and verified spark with the wire boot hanging close to the plug stem). I did note this time (when I pulled the plugs) that they were "carbonized" (dry) and looked similar to (top pic):

Dry and wet fouling
Fouling, either dry (top - matt black, sooty) or wet (bottom - gloss black, sticky),

Can be caused by many different conditions. Carbon deposits build up when the plug fails to fire correctly and burn them off. Air/fuel mixture too rich, choke stuck on, electrical problem, extended periods of low speed driving, plug heat range too cold. All should be investigated

BTW: The Motorcraft part # (plugs in vehicle, carbonized) are a "BSF42C" - copper plug.

This is one of the reasons that I tried a different plug (in place of the fouled Motorcraft plug in Cylinder #3). Note: the plugs in #3 and #4 were in the same condition, which at least gives me an indication that they are "firing", but will spark test just the same. To answer your question on milage of engine: 103K (utility truck, used only in summer for towing horses).

At work now, but will see what I can accomplish this evening (and get back with you).


Customer: replied 8 years ago.

Got stuck at work last night, so no work on truck. Got to it tonight, and ran a compression test (dry), with the following results:

#1 - 140 lbs

#2 - 135 lbs

#3 - 140 lbs

#4 - 140 lbs

#5 - 135 lbs

#6 - 150 lbs


Next I checked the spark, first on cylinders #3, and #4 (those were the weak ones that had no noticable affect when the plug wires were removed). The spark seemed weak, and the color was more like a "purple" rather than strong hot blue. Also moved timing (one my #1 mark) to 10 BTDC, and no change. The miss is still consistent, and engine running rough.

The whole time I was working on the engine, there was a strong gas smell (seems to running too rich), also recall the plugs are fouling. Undecided

ok compression is good,

igniton is good--(double check the firing order shouuld be 1,5,3,6,2,4)

when you pulled the pllugs for 3 and 4 were they wet with fuel?


Customer: replied 8 years ago.

1)I have already checked the ignition firing order, and confirmed 1,5,3,6,2,4, as for the plugs, when I pulled them (before the compression/leakage test) they were slightly wet (and carbonized from previous running; see earlier pics), indicating to me that the truck is running rich.

I am beginning to think that the (wonderful rebuilt) carb.? is either bad, or set-up incorrectly? I also did another test; I disconnected the connection to the Fuel Mixture Solenoid (on carb.) at idle speed, and it had not affect on engine behaviour (should it?), and note, the O2 senor in the vehicle was replaced 2 months ago, when the carb. was replaced.

BTW; what should the compression level be at (for a relative new engine) although it is balanced, are the absolute dry compression numbers (lbs) what are expected?


all the ford manual says is that the lowest reading must be 75 percent of he highest.

i think you may be on the right path with the carb, but why the two middle cylinders?

does not make sense... all other four were good?

Customer: replied 8 years ago.

O.K. on the compression - thanks. (just a point of interest).


Yes (on the other 4 cyl's) at least the engine "lugged" when wires removed, and the spark was there (seemed a little stronger than 3 and 4, but hard for me to tell)- this is what is confusing me. The only other thing I can think of is the magnetics on the distributor pick-up may be damaged for those cyl. ??


I think I will speak with the guy who provided the carb. to see what my options are at this time (it would be better to get a brand spanking new one if needed).


The other thought is something in the exhaust (fouled CAT?) but I doubt it, adn not sure how to check "air managment valve" or verify that the CAT is O.K. and not creating back pressure (exhaust is coming out back, and the miss can be heard after muffler)?

if it were the cat, you should have a whistling sound with high revs,

try swaping the wires for #4 and #5 they should be long enough,

is this a 'tfi' distributor or a duraspark?


FORDFXR and other Ford Specialists are ready to help you
Customer: replied 8 years ago.

O.K. - Replaced the Ignition Module on the distributor (TFI system) with a "Cheapo" ($42) just to see if it were the ignition, and it made no difference. Checked the distributor itself, and the shaft did not have any "play" in it that I could note. Could not get at pick up unit, without dismantling the entire distributor (didn't go there!). So, no real news on that front. While going over the engine to check for bad gounds and the like, I notice a rather large crack in the air tube that goes from the intake manifold to the Air Mangagement valve! Why I didn't see before I don't know. Ford was closed today, so I used some duct tape to try and seal it for now. When I did, the engine started to level out (a little). I let i run for a bit, and then checked the spark plugs...they were starting to clear up (i.e. the previous fouling was starting to clear up) so I changed them out. Then, I started to "rev" the engine up to about 2500 RPM, and noted a couple of backfires through the carb. when I rev'd and then let drop back fast. Did this when I snapped it back to idle from higher RPM, and did not do this when the fall back to idle was gradual. Did a few more times and fast, and it did not do this again. While rev'n engine, check timing, and was advancing (also checked again at idle, and was O.K.) So, having said this, I think it is about time to take it to my local Ford guy (thoughts?) and have them replace the feed tube, and check out the carburator (I have no way of checking it's integrity). I let the guys where I bought it, that it may be coming back. Oh, almost forgot, finally got the new EGR valve in, at it had no affect (as expected) but needed changing anyway).

Why are you recommending swapping #3 with #4 wires? What are you thinking?

Customer: replied 8 years ago.
Louis;Just want to thank you for your assistance, and diligence and patience in trying to help me with this one (It is a tough one! especially with an older vehicle, with OBD-1....not much to go on!). In as this issue has been going on for almost a week now, I understand entirely (definitely got my $15 bucks worth) if you need to cut it off. Just wanted to let you know, I am sensitive to you investment in time on this one. You have definitely been a large help to me. It is always good to have someone who knows their stuff, and seasoned with experience (in fact, my daughter boyfriend is a Ford Technician
Customer: replied 8 years ago.

Louis;Just want to thank you for your assistance, and diligence and patience in trying to help me with this one (It is a tough one! especially with an older vehicle, with OBD-1....not much to go on!). In as this issue has been going on for almost a week now, I understand entirely (definitely got my $15 bucks worth) if you need to cut it off. Just wanted to let you know, I am sensitive to you investment in time on this one. You have definitely been a large help to me. It is always good to have someone who knows their stuff, and seasoned with experience (in fact, my daughter boyfriend is a Ford Technician, locally, but lacks experience with some of the carberated, older vehicles, and this one has baffled him too!!). Thanks Much for the help, and your inputs to date!!

the reason i suggested swapping the #4 and #5 wires is if the compression is good, fuel should be equal if everything with the intake is good, therfore all that is left is spark, so if you swap a wire from a bad cylinder with a known good cylinder, and the miss moves, then you know it was the wire. on newer engines this is a very good test to use with coils

i don't have any books from 86, therefore i cannot tell you what the tube is you are describing, can you post a pic of it?


by the way tell "the boyfriend" that i believe that ford will be cutting training, so it would be in his best interests to get as much done as he can now, it also opens up a lot of doors.


and if you think i am doing a good job you will have a chance to leave positive feedback, (which does not happen as often (percentage wise) as negitive)--but thank you for the vote of confidenceWink

Customer: replied 8 years ago.

Here is a pic of the "Air Check Valve" It connects (from what I can tell) from the air management valve (that big stacked valve that mixes exhaust gases and Air Pump?) to the intake manifold, just under the carb. (can't paste image, but here is link)


Btw: I have already posted the feedback...which is Very Postive.

Customer: replied 8 years ago.

Forget the link.....Will try the wire swap on cyl'sgraphichere is Air Management Valve it connects too- threaded side of Check valve (above) connects to intake manifold (just below carb.), and the fluted end connects to top of Air Management Valve (below). The exhaust gases as connected to lower connections of AMV. The two diaphragms are controlled by separate individual solinoids, located near drivers side firewall.


It's getting late here on the east coast, so I will check back with you. Will swap spark plug wires on #4 and #5 tomorrow (but I have tried a whole new set of wires earlier, but worth a shot, stranger things have happened). Food for thought; the void in the air tube (check valve) going into the intake manifold is located close to #3 and #4 intake as these are the first two cyl to be fed coming into the head, from the carb. (and 2, 5, 1, 6 in this order order of distance from where the carb is mounted on the intake manifold), could this explain the behaviour of the lower performance from cyl #3 and #4 (i.e. fuel/air mixture could have been/be varying due to the vacuum leak from the Check valve sucking in "raw" air, first into these cylinders?)?

the valves you have pictured here are for air management, they will be plumbed into the exhaust, the top one is a check valve, the bottom one is tab/tad valve - it controls the thermactor air, tab=thermactor air bypass, tad=thermactor air diverter

this air goes into the exhaust at the manifold, at the catalytic converter or dumps it to atmosphere, this should not affect the intake, or missfire

Customer: replied 8 years ago.
Thanks for the info., I will get back with you once Ford has had a look (I am told that they have an "old timer" there, which may be familiar with carb. set-ups). Hopefully they can get to the bottom of this. Will let you know what I heard. But the vacuum leak on the check valve tube (was about 2 inches from the threaded nipple) was definitely "sucking clean air" and screwing up the idle.