Greetings all,

I need some help diagnosing a problem with my truck that has gone from very intermittent to happening fairly regularly. Most likely because I'm driving the truck full time until I have a spare weekend to replace the clutch in my SHO. But that's a whole 'nother ball of wax... This is on an 85 F-150 with a 300 six and 4 speed tranny.

The problem is that the truck will start to lose power and then ultimately hesitate / jerk / stutter. It happens mostly after I'm on the highway at constant speed. It will do it once, and continue unless I back off the throttle and slow down. During its fit I notice the temp guage will climb. It usually sits between the N and O, but will rise to about half way on the guage, around the M. It will usually recover and drive normally for a while (temp goes back down), then start the tirade back up after a longer while. The number of cycles really just depends on how long I drive the thing. The jerking is violent enough to pop the PCV valve out and / or nipples on the vacuum tree off.

The fuel pump and filter are new, as is the TFI module, and the PCV valve. The timing hasn't changed (set at 10* at idle). It pulls good vacuum at idle (18 to 20 inches if memory serves correctly). The one part I am suspect of is the stupid electronic 1 barrel "carb" it has. It is about 2 years old, after having to buy a complete new one from O'rielly, to replace the original which was falling apart (screws on the under side had stripped out). I'm wondering if it could be leaning out, to the point of pre-detonation, then recovering for a while. Anyone have any other ideas? I'm at the point where I want to back date to a "real" carb, even if it's not ultimately the problem. What years would work, and would I have to swap the intake too?

Thanks in advance for any and all input!

Justin Schick
jschick@aafp.org

 

Have you had the exhaust system checked? That sounds like strangely like a plugged catalytic convertor. My truck did exactly what you are describing.

Test: Have someone place their hand near the opening in the tailpipe, see if they fel any air movement from the exhaust at idle. Then rev the engine up and see if they feel any drop in pressure.

The cats in my truck had plugged so severly that if you put your hand over the exhaust pipe at idle, you'd feel a little bit opf pressure, but after revving the engine, you'd feel the pressure drop to almost nothing.

 

Good suggestion, but I don't believe this would be my problem. Mainly because the previous owner gutted the cats...



Keep the ideas coming, and thanks!

 

Ok, I've got some more info. Inbetween loads of laundry and dinner I got to play with the truck some more.

Actually, it started on the way home from work. I had a thought about testing to see if it was electrically related or not. As it heated up and started its first round of losing power and ultimately missing / stuttering / choking / whatever, I waited for a sraight stretch of road, put in the clutch and turned the ignition off. I then turned the ignition back to on and released the clutch to get the engine going again. I noticed an immediate improvement to the problem.

After I got home I decided to test my carb theory some more. I unplugged all 5 electrical connectors going to the carb. To my amazement, it started and ran. So I decided to go for a test drive, and unfortunately the problem was still there. HOWEVER, I was again on a stretch of straight road and again tried the off / on theory. It DID NOT seem to fix things like it did previously.

I pulled over and reconnected everything, but by that time was almost back home and on city surface streets again, not wanting to venture back out on the highway. Anyway, given the above scenario tests out (IE I'm not imagining things), how could it be anything else BUT the stinking Carter YFA 1bbl carb?

BTW, what exactly do all those connectors do? The only two that look remotely familiar to me are the throttle position sensor and the electric choke. There's also one going to a cylindrical device back by where the throttle linkage attaches. There's another on a valve type thing in the vacuum hose between the carb and the carcoal canister. And there's another on a vertically mounted cylinder thing attached or imbedded to the driver's side of the carb itself.

Thanks in advance for any ideas you may have!

Justin Schick
jschick@aafp.org
85 F-150 4X4 / 300 / 4spd (work horse and fill in when needed)
92 Taurus SHO (daily driver when it's not broke too)
66 Mustang GT (drag toy that I should probably start driving daily)

 

from my expierence, the temputure gage is not very acurate at all. As far as i know, it always stays in the same place unless getting realy realy hot. I installed a set of aftermarket gages and they are the only way to go.

 

That feedback YFA is nothing more than a steaming heap... Dump it and the TFI-IV ignition in favour of a non feedback carb and Duraspark II ignition, say 1982 era. The feedback carb controls the mixture somehow, and it's got an O2 sensor in the exhaust manifold and all kinds of fun stuff...
1982 F100 SWB
351w HO, 9.5:1 compression
4bbl, Hedman Hedders
Purple Hornie header mufflers
NP 435/2.75 geared 9"
1985 F150 HD, 300 HD/needs a NP 435/4.10 geared 8.8
70,000 miles
1980 F100 Custom, 300/Np 435/2.75

 

Have you pulled the plugs to look at them? Back when I had my 300 six in my 85 I had something similar happen. After spending about $400 on rebuilding that POS carb (which it did need rebuilding) the mechanic finally figured out that the gasket for the choke pulloff had a tear in it and was sucking air. This cleared up my troubles but I doubt the gasket is your problem unless you've had this carb apart. The reason I asked about the plugs is the choke would never pull off like it should have. It ran fine cold, but as the engine warmed it was too rich and ended up fouling the plugs.
At least it won't cost ya nothing to look.......

Ron

 

Yes, I really want that carb off there too. I'm thinking of going with an adaptor plate from Clifford Peformance to enable me to mount up a Holley 2300 series. Would that be any more or less straightforward than swapping parts from eariler trucks?

I am going to have to re-check the clogged exhaust theory once I can get a friend to help. Kinda hard to be at the tailpipe and gun the engine with only yourself doing the work! At idle there's a steady pressue coming out but I couldn't tell what was happening while I stood on the passenger side pushing the arm on the carb and watching the tailpipe.

What makes me wonder is a trend I noticed with the PCV valve. Once it starts running bad it'll shoot it (and usually some oil onto the firewall as well) out of the grommet at the rear of the valvecover. I decided to just leave the PCV valve out this morning after I got to work, and over lunch it seemed to run better. I also notice that when the valve is out a lot of steam-ish looking stuff comes out of the valvecover even at idle, but really comes out when I raise the RPMs up. If the exhaust was potentially blocked, could that be exhaust gasses finding their way out via a different route?

One more thing on the PCV valve. When it is out of the valvecover, I don't hear it clicking. With the engine not running I can shake it and hear it rattle. Is that normal?

Thanks,
Justin

 

The reason you're PCV is popping out is because you have to much pressure in the crankcase. You're on the right track when asking if the exhaust has to find a different way out, and the answer is yes, and it sounds like its doing this by blowing pass the rings. That is also why the "smoke" increases so much as you rev the engine. You really don't need a friend to help test the exhaust. Just get a piece of cardboard and tape in on to the pipe and bend it over the opening. Watch how it performs as you are reving the engine.

It could be a stuck EGR valve that is causing part, if not all, of your troubles. Locate the valve and unplug the vacuum line from it, then plug the line itself. Try it out and see if it runs any better.

The connections to the carb are as follows: throttle position sensor and the electric choke (those you know about) the one that leads to the canistor is part of the o2 system.

The one that goes to the throttle linkage is known as the coast control valve. Its function in life is to keep the engine rpms up when you shift or coast. The reason for this is that when you release the gas pedal while coasting the engine goes to a high state of vacuum. This richens the fuel mixture and causes increased emissions. By keeping the throttle off the idle side during shifting/coasting the engine rpms don't drop as much and you produce less emissions.

The one in the carb is the fuel mixture control vavle. It adjust the fuel mixture by reading the signal from the co2 sensor. As you can guess all of these are tied to the computer and share information to keep your truck running.

The PCV isn't rattling because at idle it has a vacuum pulling the valve closed. If you put your finger over the hole you should feel a pull on it. Durning high manifold vacuum (coasting or at idle) the PCV draws the crank case gases by sucking in air through the breather tube located in the aircleaner housing. Has you increase the rpms the vacuum signal becomes weaker in the maninfold and increases in the port (or carb). When this happens the breather no longer filters the air, and instead draws the fumes out of the engine, basically reversing how the air travels threw it.

The reason why you're going from cold to hot is because of two things. One is that your carb is reading a way rich condiction and is leaning out the mixture to the max. And the second would be a blocked exhuast that is over heating the engine.

I believe that everyone that says you have a blocked exhaust system is on the right track. The trick is to find out where/why. Check the EGR valve the way I described above also. It's poissible you might have hit something putting a bend or crease in your exhuast pipe so look over the whole system. On some enignes there is an exhaust heat flange located by the manifold to pipe connection.

This is a bimetal spring that is connected to a flange on the inside of the exhaust manifold and a conter balanced weight on the outside. Its goal is to help the engine warm up faster by restricking the exhaust until it reachs a certain temperature. If the spring has broken its possible that it has forced the flange to become stuck in a half closed half open postion.

Hope this info helps you in some way.

 

Thank you Wayne that was a very informative post! It helps immensely. I'll address your points one at a time:

The EGR (along with the smog pump, A/C and many more things) was disabled and / or removed by the previous owner. He was a real hack master (slash and burn type of home "mechanic"). The valve is still there but has no vacuum going to it, so AFAIK is inoperable and most likely not causing me any grief.

I did put my finger over the PCV valve and noticed the behavior you described. So I think it would be functioning normally if I could get the truck running right.

Where exactly is this spring and weight combo located on the exhaust manifold? I haven't noticed anything fitting that description but would like to check it out.

I'm now pretty much convinced the exhaust is clogged somewhere, too. It has two huge honking cats on it - is this normal? When I bought it a couple years back the previous owner said he gutted them, and I found he'd hacked off the tailpipe and muffler. While at the exhaust shop getting the loudness remedied I watched the tech bang on the lower one and say it sounded hollow, but I'm wondering if the one closer to the engine wasn't given the same treatment and is actually blocked. I am very close to going to town with a Sawzall and then returning to the exhaust shop...

Thanks again for all the input guys!

Justin

 

If it were mine, I would do just that...get the sawsall out and cut your pipe just before the first cat. Then see if your problem disappears. I would also junk that carb and TFI-IV as another poster said and go with a replacement carb and duraspark ignition. If you want an alternative to normal PCV system, search this forum for PCV jar and read that post. It's a homemade filter that keeps crankcase vapors/gases out of the top end of your engine, to make it run longer and cleaner.

 

Ok guys I just went out to inspect the cats with a buddy here at work.

I took a small hammer with me and before starting it got underneath and banged on the cats. The rear one sounded tinny and hollow. The front one gave a more higher pitched "solid" sound.

Then I had him start it with me back at the tailpipe exit. At idle there was slight pulsating pressure pushing my hand away from the tip. When he would rev it up, it would be pushed on a bit more, then get sucked back in pretty hard.

Would everyone agree it's pretty much difinitive that I should be saying buh-bye to the front cat very soon??

Justin

 

Well folks, it's not a clogged cat. :-(

I got antsy at work and with it being slow managed to get the afternoon off. Went by an exhaust shop that was recommended to me by a number of people (closer than the one I usually use out in Grain Valley). Anyway, I get there and ask if they can test the front cat. They say no because of a leak at the manifold, missing air pipe to the cat, etc. I say ok, how about just cutting them out? They say sure, but they have to put at least one back in. I say ok. As soon as the old two cats hit the floor I went to inspect. Sure enough, both had been gutted by the previous owner and were completely hollow. I knew then all this was a $180 mistake. End result is I have straight pipe where the 1st one used to be, a new cat where the 2nd used to be, and a much quieter truck that still has the same heat up and miss / jerk / pop the PCV valve out problem.

So, what's next? I really want that carb off of there. What are you opinions on getting an adapter plate from Clifford Performance (if they would ever reply to my e-mail) to use a 2300 series Holley with, versus tracking down a used manifold / carb / whatever else I need to get from an earlier 300?

Thanks,
Justin

 

You haven't mentioned if you checked the EFE valve. This is the sucker that's right before the exit hole of the exhaust manifold. I don't think it's your problem, but you might as well check it as best you can. It's spring loaded and the counterweight is on the left hand side. It should move towards the engine when the engine heats up - this opens the valve so that it doesn't divert exhaust gasses into the heat riser region of the intake. Check when cool: make sure the valve moves by pushing on the counterweight. Mine (on my original manifold) was frozen shut, but I was able to tap it loose with a hammer and some penetrating fluid. - low and behold 9 months later when I replaced the manifold with the correct design (proper EGR source hose) I found that the flapper part was GONE - either completely disintegrated or lodged somewhere else in the exhaust pipe.
Your problem sounds like fuel / ignition problems. Does it only happen at a paticular speed range?

 

The exhaust that is not hitting the sensor is being forced into the crankcase. In the crankcase it's being sucked back into the intake manifold by the breather tube and PCV (that makes sense as to why it runs a little better with the PCV off, because its not sucking so much exhaust back into the intake and not getting that extra fuel reading at the o2 sensor) Ok so what can force the exhaust into the crankcase.

Will there is a couple of different things. One is a blockage in the exhaust system. You've checked the whole exhaust pipe and found it to be in good shape. Looking in my manual I have found that the valve I described earlier (last post) is located right where the pipe meets the exhaust manifold. The correct name for it (in the book I have anyways) is the exhaust heat control valve. Just get under the hood and follow the exhaust manifold and it should be there.

If that valve is stuck closed it will restrict the exhaust to the point where it will be stuck in the cylinders and forced passed the piston rings. Any exhaust that does make it pass the valve will be in high concentrations and give the o2 sensor a false reading leaning out the system and making her run hot (also its trapping all that heat in the exhaust manifold not helping the situation any).

What’s going on out back at the tail pipe is an example of how gas follows in a restricted situation. When you’re idling the gas has filled the area in front of the restriction and what little that is being allowed pass you feel has the pulse at the tail pipe. When you rev the engine more is forced passed the restriction. When you release the pedal there is a pressure differential between where the gas is restricted (high pressure) and the tail pipe (low pressure) and your hand gets sucked in.

There is one other possibility that isn’t so nice as the restricted exhaust. If you had a burnt head gasket that was leaking into the crankcase it would act much the same way. Though you should also have a problem with burning oil, but that’s not always the case.

If you have a vacuum gage that could help in diagnosing the troubles. Here is a little chart that tells what the readings mean.


Smooth and steady idle(800 to 1200 RPM) Between 17 to 21 inches Engine is in Good Condition, but perform next testto be sure.

Open and close throttle quickly Jumps from 2 to about 25 inches Engine is in Good Condition.

Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next testto be sure.

Open and close throttle quickly Jumps from 0 to 22 inches Confirms worn rings.

Steady idle Intermittent dropping back 3 or 5divisions and returns to normal Sticky Valves. If injection of penetrating oil intointake manifold temporarily stops pointer fromdropping back, it's certain the valves are sticking.

Steady 3000 RPM Pointer fluctuates rapidly, faster enginespeed causes more pointer swing Weak valve springs.

Steady idle Fast fluctuation between 14 to 19points Worn intake valve stem guides. Excessive pointervibration at all speeds indicates a leaky head gasket.

Steady idle Constant drop Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally missfiring.

Steady idle Steady 8 to 14 inches Incorrect valve timing. It must also be rememberedthat vacuum leaks and/or poor compression canresult in a low vacuum reading.

Steady idle Steady 14 to 16 inches Incorrect ignition timing.

Steady idle Drifting from 14 to 16 inches Plug gaps too close or points not synchronized..

Steady idle Drifting 5 to 19 inches Compression leak between cylinders.

Steady idle Steady below 5 inches Leaky manifold or carburetor gasket, or stuckmanifold heat control valve.

Steady idle Floats slowly between 12 and 16 inches Carburetor out of adjustment.

Blipping engine speed Quick drop to zero then return tonormal reading Muffler is clear.

Blipping engine speed Slow drop of pointer then slow return
to normal reading Muffler is choked or blocked.


I hope this helps you some.