Hi all,

'91 f150 4.9L 2WD

three weeks ago my truck failed the AZ emissions test in spectacular fashion. They kept my original paperwork today when I went back to retest, so today's results are all I have:

Hydrocarbons: 4.41 (2.0 is standard) FAIL
Carbon Monoxide 85.70 (standard is 20!) FAIL
Oxides of Nitrogen 3.56 (4.0 standard) PASS

After the first test and before today's test, per a couple of rounds with the code reader I replaced the following:

Fuel Pressure Relay (under the hood)
Purge Cannister Solenoid
EGR Valve Position Sensor
MAP Sensor

- my CM codes also reflected a 211 Pickup Circuit fault. I did not replace this, as I wanted to see if the above replacements got me through the test. They did not, although the numbers are better than the first test earlier this month. No check engine light now, but I have a really high idle. Two years ago I replaced plugs, wires, ignition coil, and the cat - all are less than two years old; I replaced the plugs again before smogging as they had about 15,000 on them in two years and iirc that is the recommendation for the coppers?. The distributor itself is at least 10 years old(I have replaced the cap and rotor regularly, though) - and I'm going to assume that is something I would replace at this point due to age and the possibility the pickup is bad?

My somewhat loaded question is - is a bad pickup enough to give me such terrible numbers? The truck runs *better*, but something isn't right (obviously) and I get a high idle although it starts just fine.

 

Your high idle problem could be the source of your emissions problem, if the problem that is causing the high idle is a bad sensor then that could force the computer into open loop which is a much richer fuel map. So to find the source of that high problem start by disconnecting the TPS sensor while the engine is running, if this makes no difference then it's possible you have a bad TPS, because normally the computer would see that the throttle blades are closed and open the IAC valve a little to set idle speed, but if the TPS is out of range then it defaults to open loop presets and ignores the TPS. Of course a stuck IAC valve, a leaking EGR valve or broken EGR tube that lets fresh air in through the valve, or a gasket leak or a badly worn throttlebody could also cause a high idle.

The 211 code normally manifests itself as a stumble or no start but idle speed remains unaffected.

 

Interesting, okay. I replaced the TPS about four years ago, I'll try your test tho. Would there be a code with this, normally?

I did replace the egr valve position sensor, but will do the check for leaks. The idle does not appear to run so high and rough until the truck is warmed up, which makes the vacuum leak more likely, I guess. If it's cold when it starts, it sounds fine - but if it's warm (say, like I drove to the store for a quick errand, turned it off, and turned it back on) then it really races, and then settles down into a still-high idle.

The emissions station is really pushing the "faulty ignition system" thing.

 

Umm, aren't 91 F Series trucks still two-digit fault codes? I know mine is. If I'm reading your post incorrectly, what were the codes that pointed you towards replacing the parts that you noted on your OP?

 

I bought a code reader for around $30. I have short attentions span issues - the thought of the paperclip count the flashing light system made me itch.

First time around:

542 Fuel Pump Secondary Circuit Fault (I replaced the relay)
565 Canister purge solenoid fault (replaced that)
10 Cylinder balance code - dunno why that one, I only ran KOEO/CM as the instructions were to not bother with KOER until the first was cleared of codes naturally (repaired). Balance codes are only supposed to be run during KOER test.

Second time:

542 Fuel Pump Secondary Circuit Fault (I ignored that one)
211 Pickup Circuit - I set this one aside as well, for now.
327 EGR Circuit below minimum voltage - replaced egr valve pos sensor
328 EGR Closed Valve Voltage lower than expected - ditto
10,30 (still don't know what is up with that)
And based on a hunch, I replaced the MAP sensor.

I have had no CEL since Wednesday afternoon when I replaced the latest round of sensors and drove it a lot yesterday and today, freeway, stop and go traffic etc. Thinking I was good, this afternoon I went back to smog check...

 

A bad TPS can set a code but if it develops a dead spot above the closed position it may not. Also check for a sticking throttle plates or a binding cable.. anything that may prevent the throttle from fully closing. To test the EGR apply manifold vacuum or an external vacuum source to the valve with the engine idling, it should stumble and stall if there is nothing but exhaust gas getting in, but if there is fresh air getting in rpms will rise just like you opened the throttle.

 

So, that's a no. Couldn't get to it until this evening, work and all... disconnected the TPS and the engine surged right away. Just for giggles because it was running and I was there anyway I pulled the little green hose off the EGR, which produced no difference and also disconnected the idle control valve - also no difference. Truck was idling and hot as I'd been doing the last minute errands before the holiday stuff. Ran out of light so I couldn't check for any vacuum leaks (starting with the EGR) or get a look at the throttle plates, hope to have time to get to that tomorrow.

FWIW, I also noticed the freaky high idle is worse when the clutch is depressed.

 

Although I try to do all my own work, when I bought my truck, the engine light was on, and because it hadn't been emissions tested for some time, I had 30 days from the time I registered it to git'er done. I brought it to a garage which does emission work and they charged me $150, replaced no parts, and used a lot of that black sealant stuff.
Engine light went out, and truck has passed up-the-rear pipe emission testing twice. After the next test in 2016, truck will be 25 years old, and therefore exempt from future emission testing in CT.
I hate emission testing. It's very stressful as the date approaches, and if there is a problem, it can be a @#$%ing nightmare. I think I got extremely lucky...

 

There is no "Zero" with the old EEC-IV system. What you are seeing is a "1" -long pause- and "3".
3= 6 cylinders. After you get the 3 beeps, you can enter different test mode by pressing the accelerator, toggle the overdrive, toggle the A/C switch, ETC.
The 1 is to indicate the end of the hard codes (faults that are happening right now) and the beginning of the stored code (codes that have happened in the last 50 starts).

Id check for vacuum leaks first. Spray some carb cleaner around the intake gaskets and listen for an RPM change.

 

Well, at this point I am 99.9% sure that there is a bad exahust manifold gasket in there... Maybe. Possibly. Got home later than anticipated, went over the truck spraying throttle body area, egr valve pipe, assorted hoses, that funny little vaccuum tube that runs under the intake manifold to no result. Laid behind the passenger front wheel and sprayed as close to up underneath the exhaust manifold and over the rear downpipe donut as I could get. Definately got a hit in the form of an immediate stumble from the engine when I sprayed whatever it was in that area. Since there doesn't appear to be much more than the exhaust there I figure that is most likely it (I did double check to see if it was the valve for the air pump in the spot and came up fine). It also doesn't look like something I am even remotely interested in doing myself. I see that it's a fairly inexpensive part - any ideas of what I can expect typical labor hours on that job to be?

 

Your truck uses 2-digit codes. Your code reader seems to be getting confused, and thinks they're 3 digit codes.

 

1991 was the first year for 3 digit codes. Since the codes seem valid I suspect they are correct.

 

Some applications. I'm pretty sure the '91 4.9 I've worked on used two digit codes, but it may have been that specific combinations.

 

My 91 F150, 5.0, M5OD, throws 2-digit codes. Maybe some that were manufactured later in the year started using 92+ EEC-IV computers. My truck was manufactured in late 1990.

 

I should have been a bit more explicit. Ford started to transition to 3 digit codes in 1991. Trucks w/E4ODs were the first to use them. Manual transmission trucks started later.