Hello, I'm a new user:

I'm experiencing problems with my '94 F-150 4.9-6 as follows: Engine is hard to start when cold (first start of day). When engine is cold, truck stumbles severly upon acceleration. If I tread VERY lightly, the drop-off is minimal, but any pressure on the accelerator will cause the truck to stumble badly. At interstate speeds after the engine is warm, the truck will surge, stumble, and/or miss upon acceleration. Occasionally, the truck will buck even on deceleration. The problem doesn't seem to be moisture related; only operating temperature seems to change the condition. The problems never go away; only lessen with prolonged engine warm-up. The engine idles fine whether cold or warm. Upon cold start-up, it works through a seemingly normal fast idle-to-slow-idle sequence.

I've replaced the plugs, wires, cap, rotor with no effect. I tried to code out the engine with inconclusive results. I've replaced the transmission filter and fluid, and am convinced that the transmission is behaving fine; no slipping or erratic shifting.

I'm at a loss as to what to try next. I don't want to throw parts and money at the problem; I'd rather pinpoint the problem and fix it, or at least narrow it down to a few likely candidates. This is my first Ford truck, and I like it very much except for the stumble problem. If anyone can shed some light on this problem, or has experienced this in the past, please respond. I appreciate your help.

 

[updated:LAST EDITED ON 16-Sep-02 AT 11:17 PM (EST)]A couple things come to mind on what your talking about. First, when you changed the cap and rotor, did you notice any oily residue on the pick up coil? Sometimes, the dist. shaft will wear the bushing out of it and allow oil to come up around it, shorting the pick up signal. Next, I'd sweep the TPS slowly with a DVOM and see if you have a flat spot {when voltage fluctates rapidly} in the TPS. Also when was the last time the fuel filter was changed?

 

Also check vacumn connections. Listen for hissing. Take a small paintbrush and swipe water onto the hoses to see if there is a leak.

 

Thanks for your feedback. I didn't notice any inordinate residue on the cap/rotor when I changed them, but I'll check this again to be sure. If the shaft is worn letting oil into the cap, is there a re-bushing kit available from the dealer or auto parts stores, or is this a throw-away item?

When I check the TPS, am I looking for a specific voltage reading, or just checking for consistent resistance?

I haven't changed the fuel filter since I've owned the truck; no idea before that. I'll pick one up today. Do you happen to know if there are two filters on a dual-tank set up?

Thanks again.

 

Thanks for your help. I'll run down the vacuum hoses with soapy water and see if I can find the culprit.

Thanks again!

 

re: fuel filter replacement... I've tried this myself and failed miserably. I couldn't get the dang connector off. You might have better luck.

A tip: remove the fuel pump fuse, then start the engine and let it run til it dies. This removes pressure from the fuel system so that you don't get sprayed with high pressure gas when you remove the filter.

 

Gave changing the fuel fiter a shot last night; couldn't get the connections loose. I believe I have the proper tool to disconnect the garter spring retainers (K-D tool specifically designed for this type of lines). I've used these tools before to disconnect various other Ford and Chrysler fuel and A/C lines with good success. In this case, the "nose" of the disconnect tool seemed too short to engage the internal garter spring, and release from the flange on the filter nipple.

Question: is there another tool that is specific to this type of fuel filter connection? I don't have a problem buying any tool that I need. Thanks for your help.

 

[updated:LAST EDITED ON 20-Sep-02 AT 09:18 AM (EST)]Changing the filter can be a P.I.T.A. Try a little wd-40 or equivalent in the connector and clean them out best you can. If you need to, you can pull the springs out with a scribe and replace them. {Parts stores or a Ford dealer have them.} You are looking for a spot in the tps where voltages sweeps up and down erractically. It should read voltage steadily as it goes up. The range is from about .92 volts {at throttle closed position} to 4.5 volts, at W.O.T. There is a bushing kit for the dist. also at your Ford dealer or parts store. Good luck.

 

Thanks for the continued feeback.

I looked at the vacuum lines last night, and relpaced the one that goes from a rectangular canister near the right front frame rail to over the inake plentum, branches in two, and connects to the base of the throttle body on top; No difference. The others looked o.k.

I checked the TPS with a DVOM (on the ohm setting) and didn't see any erratic movement. The numbers swept up smoothly with the throttle movement. Connected to two of the three terminals, the numbers started low, and increased as I opened the throttle; Using two other pins, the number started high and decreased as I opened the throttle, and connected to the remaining two pins, the numbers didn't change at all. This is with the harness disconnected. You mentioned voltage readings; should I leave the harness connector in place, turn on the key, and back-probe the connector using the voltage setting on the DVOM?

I don't really suspect the fuel filter because the engine dosen't seem to be starving for fuel. On the highway, when the truck starts to stumble (in O.D.), I can manually knock it down into 3rd. to increase the RPM's and the engine smooths out. If it were fuel delivery-related, increasing the RPM's should make it worse, right? The problem is the worst when the RPM's are low, and there's any load on the engine; a slight grade on the highway is enough. When the engine's cold, just trying to move itself makes it fall flat on it's face.

How about one of the many sensors/switches screwed into various places on the engine? Some of these appear to go into the water jacket and switch vacuum or voltage relative to temperature. Since this thing is very temperature-sensitive, could one of these be bad?

I also noticed a couple of connectors on the wiring harness that aren't connected to anything. One is near the front fuel injectors, in the same harness as the fuel injector wiring. The other is near the rear of the left side of the valve cover. There are two gizmos mounted to a bracket there, and the bracket has an open spot for another gizmo. The wiring connected to these two gizmos come from the same place in the harness as the suspect connector. Am I missing a gizmo, or is the open spot in the bracket and wiring connector just there for a better-optioned truck, different engine, etc? I don't think these are spouts, test ports, etc, because I found a few of these under the hood, and they had caps on them, and were labeled.

This thing's downright dangerous to drive in traffic when it's cold. I'm going to have to get this straightened out pretty quick, or I'm trading it for a new truck before I get hit; I need something that will GO when I press the pedal. It's a beautiful truck in like-new condition, but right now it about one step above a two-ton paper weight in usefullness.

Thanks for your help...

 

OK, this might be similar to what happened on my '92 4.9L 6...

When cold, the truck really bucked. It would sort of cut out for a beat or two, then come back to life. I could really floor it and it would 'get past' the problem, but when just cruising steadily along, it would surge and buck.

Nitro94 suggested a fix, which I implemented about a year ago. It turns out that the EGR sensor on these engines forces a lot of exhaust gas into the engine which leans out the fuel mixture too much (blame the Feds!). The EGR is on the back side of the intake manifold for the engine. It has a vacumn hose and an exhaust line attached to it.

First, to diagnose: remove the vacumn hose from the EGR sensor, and plug the hose. Then drive the truck for a while (the Check Engine light will be on, but ignore it). If the truck is noticeably better, then this is the EGR problem.

Second, to fix: get a sheet of aluminum at a home improvement store. Get two EGR gaskets at your local friendly Ford dealer or parts store. The gaskets will have two big holes in them. Trace a gasket onto the aluminum sheet and then cut this form out of the sheet. Don't cut any holes yet! Now trace the holes on the gasket onto the cut out aluminum form.

Remove the bolts that hold on the EGR valve. You'll see that the EGR valve has only one of the holes open on the 4.9L (on mine, the upper hole is the only one open). Take a 1/8" drill bit, and drill a hole into the aluminum form's traced hole. Now you have a 'restrictor plate' that will restrict the exhaust gases passed by the EGR.

Sandwich this aluminum restrictor plate between the gaskets, put the sandwich between the EGR and manifold, and bolt it all together.

This fixed years of bucking (and 5 dealer visits - none of which solved the problem). The truck began running like a new vehicle. Smooth as silk. Even my 11 year old daughter said the other day "gee Dad, the truck's so smoooooth these days!".

And it passed NJ's very strict emissions inspection with flying colors.

 

Thanks for the feedback. I'm getting ready to do a 150 mile road trip for business today, and I'll try disconnecting the EGR to see if it helps. If that's it, shouldn't be a problem building and installing the restrictor plate (fist time I've regarded restrictor plates as a good thing!).

We don't have emmissions testing in San Antonio yet, so I'm thinking about just plugging the hose, and pulling the bulb. If we get testing here, (they're talking about it) it'd be simple to hook up the hose long enough to "pass the test". This would eliminate ANY exhaust gas in the fuel mixture. Whaddaya think?

Thanks...

 

Sounds like a plan.

One extra caveat. I've heard that if you disconnect the battery for about 10 minutes, the truck's engine management computer will loose its memory. Then, when you reconnect it and start up the truck, the computer will re-learn its driving characteristics.

If you've unhooked the EGR vacumn hose and plugged it, the computer will re-learn the fuel mixture without the exhaust gases coming in, and things will be even better than if you didn't have the computer re-learn. At least that's what I did on mine, and it worked great.

 

real quick here since I have to get going...

Well, I've heard from some people that disconnecting the EGR's vacuum line didn't change one thing for them, so I'm thinking the EGR can get stuck open sometimes and the presence, or lack, of vacuum may have no effect in these cases.

The EGR solenoid would be suspect, too.

Off all of the vacuum lines that might make trouble, the one that vents the rectangular box probably won't affect the truck much. (but you can bypass the vent for a bit (and connect the two "Y" lines together) to test if it is the vacuum switch in that line).

Check the gasket for the EGR - um... i guess you could pour water over that area while the truck is running to see if it gets sucked in... or maybe that would be too hard to see... I dunno. Maybe the truck would run different.

Generally, the EGR restrictor plate works for the stumble off idle for 4.9L manual tranny users, but a stumble while going down the road sounds like a different problem to me.

just my 2 cents.

Best,

 

I followed up with all the suggestions, and still no luck. So I finally broke down and took it to a mechanic friend of mine. Coil was bad; it was putting out less than 30,000 volts. Replaced the coil - votage went up - problem fixed.

I'm going to go ahead and do the EGR restrictor plate thing, though. Sounds like a good idea.

Thanks...