I'm new to this forum and have checked back a few months worth of archives, but haven't found anything similar. I have a 92 f150 with 4.9L EFI, 5 speed manual, 70k miles. It has a slight hesitation or miss under light loads (flat or slight incline) with light gas pedal pressure, at 25-40 mph. I don't notice anything at higher speeds. All new ignition components (plugs, cap, rotor, wires, coil). New fuel filter also. I'm pretty sure it's a fuel or EEC issue, but I'm not sure what to attack first. I was told by one backyarder that the throttle body plate probably had carbon build-up inside, and to spray a bunch of carb cleaner thru it while running at high RPMs. A cursory examination didn't reveal any carbon, and his suggestion didn't change anything. Any suggestions?

 

There are a few other people posting (myself included) who are having a similar problem with the six. I would ask what your fuel economy is like? Has your mileage suffered and do you have a slight, occasional miss at idle? I have narrowed my problem down to 1600-1900 rpm at the same speeds you are having trouble. Also on flat or very slight inclines. It goes away when you tip in more throttle. I have a 90 and have had the problem for about 3 months. The computer produces no codes and I have checked all sensors individually to make sure they work. My hunch is that it is a bad ground or a vacuum leak in one of the smog lines. I have not had the time to look further as my wife is terribly ill. Let me know if you have any other ideas or have found the problem.

 

I thought I had cured all my misses with the new wiring harness between the relays and the engine. But I still have something similar to yours. Most noticable on flat smooth roads, about 30mph, just easing along and for several minutes about the time the thermostat is getting ready to open, it gives me a surging little miss. Once it gets plenty warm or I get into the throttle, then it's gone. It's done that since the day I picked it up new. It's a 1991 F250 4x4 w/300 six and ZF tranny. All the goodies, air, cruise etc. It has a new wiring harness between the drivers fenderwell and the engine. New distributer, cap, rotor, ignition module, coil, MAP sensor, temp sensor, O2 sensor, thermostat, relays, plugs, wires, alternator, exhaust from the manifolds back, fuel filter, air filter, and I am sure there is another $500 or so in parts new under there somewhere and it still does some minor missing. At least the stalling, bucking and missing is gone. The wiring harness fixed that.

And you know, my 1988 F150 was almost the same truck and I never ever had a miss in it.

Carlos

 

I had the same problem with my 1989 4.9. It would miss under light load. It got so bad it felt like it was surging when going down hill. After doing a lot of things to try to narrow down the problem I just lifted and plugged the vacuum hose going to the EGR Valve. The truck ran beutifully after that. I realy haven't fixed the problem since I think Its the EGR relay thats the culpret. I read 45 ohms across the coil of that relay but the two next to it read 90 ohms. I'm going to eventually replace it but its so very cold hear and the truck runs so great now I'm in no hurry. I understand the EGR sends from 10 to 25 percent exhaust back into the intake to cut down on NOX emitions. Can you imagine it constantly giving you 25 percent exhaust on the intake. You would miss too under those conditions.

 

After you have replaced part after part and this problem is still there try a simple restricter plate to limit the EGR flow.

 

Thanks for the feedback, everyone. In lieu of swapping out a gazillion parts on a blind hunt to narrow down this nagging little miss, I think I'll try the suggestions relating to the EGR (again thanks to Hanky and Nitro94), and I'll post soon to let y'all know what happened. To "youngster", yes my mileage sucks. Getting about 12-14, and that's mostly highway. I'm sure I was getting over 16 before this condition started. I hope your wife gets better! Try the EGR ideas when you get a chance. Maybe this will work for carlos57 as well...

 

In reference to my 92 f150 with the hesitation/miss at slight throttle pressure, removing and plugging the vacuum line to the EGR got rid of the hesitation, but of course the check engine light comes on because the EEC thinks it's broke. You mentioned a restrictor plate...I assume you mean completely blocking the opening between the EGR and intake? Or are you advocating just partially blocking it? If so, how small of a hole in the plate? And would replacing the EVP or EGR valve help, or is this condition common with this system and your suggestion being the simplest and or cheapest fix? How did you come across this simple idea? And is an EGR really necessary on a vehicle with a catalytic converter anyway? Thanks in advance, assuming you have time to answer!

 

You need some EGR flow so remove the valve and use the old gasket as a template. Trace outline on a piece of metal, cut it out and drill one hole same size and the other one about an 1/8 in. Reinstall with gaskets on both sides. This will reduce the flow and still allow the alve to do its job. What I have been told is the cause is normal engine wear and the inability of the computer to adapt for it. Most all 4.9 engines and some 5.0s will experience this sometime after 40k.

 

I had a similar problem on my 93 F-150, problem turned out to be bad fuel injectors. The local dealership replaced 4 of the 6 and boom, no more problems.

 

I had the same hesitation/bucking problem on my '92 F150 5.0 with 6 speed manual. I have done a number of things---replaced both fuel pumps because of low fuel pressure and intermittent operation---replaced the O2 sensor---replaced the fuel filter. But I still had some hesitation/bucking at a slight throttle about 35-45 MPH. The EGR relay was working and the EGR was switching. I added a restrictor to the EGR (between the EGR and the intake) by drilling an 1/8 inch hole in a "homemade" restrictor (~3/64 aluminum sheet cut to fit). After installation, the hesitation is gone---but the true test will be this Sunday when I go back to Arlington from Waco.
I'm having another problem (have had for some time) concerning the idle speed. Seems that it is not wanting to idle like the idle speed has been adjusted down. That is my next project. I'll post when I get back from Arlington to let y'all know how the restrictor plate works.

Gembeau

 

I'm having a very similar problem with my 91 F150 302. I have replaced plugs, wires, distributer cap, rotor, TPS, checked timing, changed fuel filter, and checked fuel pressure. I still have hesitation when the engine gets warm, and when under slight load I get some light knock. There are no noticable vacuum line problems, but I will go over them again and maybe now try the EGR restriction. I'm beginning to run out of ideas.

 

Gembeau,
How did the restrictor plate work out?
Fish

 

[updated:LAST EDITED ON 10-Jan-02 AT 11:35 AM (EST)]I finally got a VERY experienced Ford dealership mechanic to talk to me about this issue. He said we're on the right track with the EGR. The reason it starts happening as the truck ages is because of "coking" on the valve seats (carbon buildup). This, coupled with the introduction of exhaust gas from the EGR, changes the air/fuel ratios just enough to cause the stumble. "You didn't hear it from me" he said, "but if I were you, I'd disconnect the tube between the exhaust manifold and the EGR, put a wadded up ball of tinfoil in there and reconnect the line. The foil will block the flow but allow the valve to function, so the computer will still think everything is working properly, and you won't get a 'check engine' light." He also said that because the 4.9 runs so lean, it doesn't really need EGR anyway. This is also why the 4.9 shouldn't ping after the "operation". Some V8's might get some ping--if they do, then restricting the flow as Nitro94 and Gembeau suggested above should allow just enough EGR to get rid of it. Of course, if you really want to eliminate the cause, you'd have to have the valves ground to get rid of the coking. But that's a whole lotta work when good old Reynolds Wrap will do the trick. By the way, it worked for me!

 

So is the tinfoil moving in the tube to induce a vacuum? Is there some seepage past the foil, or did he mean to block it entirely?

I'm still thinking about doing the plate thing with either a 1/4" hole drilled, or a 1/8" hole.

 

My'93 5.0 did the same thing. I made a plate out of tin with no hole in it to go between the egr valve and the manifold and it cured my problem. Why bother feeding spent gasses back into the engine? The valve and sending unit work fine but the gasses just can't get there any more. SR.