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Having some intermittent idle issues with my 1984 F150. It has the 300 Straight Six with a mostly stock ignition and fuel system (EEC IV/TFI, Carter YFA feedback carb).
Symptom: sometimes it idles very low to the point of stalling (especially if the A/C is on and in gear, no surprise). I misdiagnosed this as a throttle kicker issue (which uses four wires and is ECU controlled, not a single wire A/C only kicker), and have verified that the new kicker is indeed working.
I’ve rebuilt the carb, and adjusted the idle mix screw as Eric demonstrated in one of his videos (done during a warm idle that wasn’t low and messed up, adjusted for max vacuum). I also replaced the fuel line and filter for a bit of hose and a clear Cartridge-style filter since the old line went directly over the engine and was shortened with a bad flare job (trying to catch/counteract heat soak). The hose does get hot (carb and mechanical fuel pump are on opposite sides of the engine) and have seen fuel boil off; however, I’ve also had the idle issue with the filter full of liquid so I am lead to believe that such isn’t the issue.
The choke is also modified a bit. Originally, it used a dual-heat setup (like Eric’s original Fairmont engine). However, when I bought the truck last month I found that the heat tubes were rusted away so I Capped/plugged the lines to prevent a vaccum leak. If anything it seems like the electric half of the choke opens too quickly. I kept the original adjustment from before I rebuilt the carb (indicated with paint marks)
Also, it seems like the jump between the fast idle (only on a very cold start) and the next step (supposed to be four steps, but I only ever see two of them. The first is screaming fast (no Tachometer, but sounds like over 2000RPM), but as soon as I tap the gas it drops a lot lower. During a long (ish) trip today it went from bad idle, to working fine, to bad again all within a 1 hour drive.. so I think this “issue” isn’t related since all of that was at operating temp. Usually the issue is at operating temp, although it happened this evening with a cold carb and warm engine (had the hood up while swapping the throttle kicker and doing other unrelated work).
When throttling up it usually does fine… occasionally it lugs but letting off the gas and punching does the trick.
Ignition: replaced the cap/rotor, wires, and plugs (all needed replaced in a very bad way), used stock motorcraft copper plugs. Replaced a failed (and incorrect) TFI module and repaired some messed up control wiring. This corrected an intermittent no-start condition. Due to one of my timing lights being on “permanent” loan and the other MIA, I haven’t set base timing yet. As I type this I realize that this might be more of an issue than I thought???
Engine health: It has 102K on it, one cylinder might have a slight amount of blow by (based on the intermittent puffs of oil vapor from the vent tube when the hat is off). When I swapped the plugs all looked healthy (medium-dark brown). I haven’t done a compression test or leaked anything down (I have no tester or compressed air). It does sometimes try to diesel on power off (I assume this is somewhat normal for a carbed engine, right?).
Suggestions on where to look next?
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Went ahead and fetched an EEC IV reader at the local parts store. Among other things, it had faults for running lean, which I confirmed is an active fault.
From there, I verified the lean condition by forcing a small vacuum leak, which killed the engine. I then restarted and used the carb cleaner trick… and found that I have a large leak at the carb base gasket.
I suspect my issue may be that I stacked two gaskets to try to reduce heat soak (something I read online would help with such). What happened was my carb kit came with a thin gasket, but not knowing such was included, I ordered such separately. The old gasket and the new separate gasket were a lot thicker, which I read provided additional insulation between the intake and carb. I also read of people stacking gaskets to improve such… maybe not such a good idea in retrospect.
Once the motor cools down a bit, I’ll pull the carb off and see what’s going on.
Pulled the gaskets. They aren’t stocked locally, so I did my best to peel off the thin paper gasket without messing up the thicker lower gasket. Also replaced the lower adapter gasket that I bought but never installed. No immediate change.
My next move is to obtain a timing light (borrow or buy used for cheap, since I have one already that’s lost at my parents’ house). Perhaps timing adjustments by the ECU are putting me in and out of time due to the base timing being off. Otherwise, barring some unexpected carb issue (odd, I just rebuilt it), my only other thought is bad gas, but that’s grasping at straws.
Ideas are welcome… these aren’t my forte… yet. I bought the truck partially to learn old systems, and learn I will.
Obtained a timing light. Discovered that my base timing was WAY off (yes I disconnected the SPOUT plug)… fixed that, but no change to symptoms. The symptoms have evolved to a mostly consistent low, rough, and unstable idle which often stalls out when in gear (and sometimes when not, even with the A/C off). Bad enough for me to park it and drive my other truck instead 🙁
I then ran a fresh KOER test. I received codes 12 (idle too low), 13 (idle too high), 23 (TPS out of range, may have been my fault for not mashing the gas at the right time), 41 (O2 sensor reports lean), and 73 (another TPS code, likely due to my goof on the throttle hit).
From this, I’m suspecting either:
- I still have a bad vacuum leak
- The carb’s idle circuit has an issue, despite being rebuilt
- There is a partial misfire that is not burning all the fuel (shows as lean due to unused O2)
- The O2 Sensor is faulty and actually making it run way too rich
My fuel economy is garbage too… if that makes any of these more likely.
Also odd is that the RPM range was faulted as both too high and too low. If this wasn’t an idle-only issue, I’d suspect a distributor pickup/wiring problem. However, I seem to be getting consistent spark, so I doubt that’s it.
I took a video of it idling… but the video doesn’t seem to show the “roughness” well. Also, something in body is knocking from the rough idle… which made it sound like I’m about the throw a rod (not the case!). If this helps, I can upload it to youtube anyway.
I was thinking the same thing… but I don’t have one… or do I? Read below:
New gasket came in. Swapped it, no change. Bummer. The “old”gasket was sketchy though…
Then I remembered that I had a cheap fog machine in my garage that I bought for smoking intakes… and with the magic of spare fuel line and electrical tape was able to make it fit one of the vacuum lines (in this case, the one that goes the the intake selector on the filter hat). Smoked it… and surprise, there’s a massive rotted vacuum cap on the vacuum manifold that’s gone wide open. I couldn’t see that the end was popped off due the PCV line blocking most of the view. No ****, sherlock. Doh!
After replacing the bad cap, smoke testing showed no more leaks… started it, it idles… barely. Since we’ve ruled out a vacuum leak (plugged the testing line with a vacuum gauge) and my vacuum was quite weak, I decided to try opening up the idle fuel screw… voila. Way, way, way too lean… got it to a happy spot (hard to dial in due to the carb feedback auto-adjusting the mix)… Good vacuum, and good idle with or without A/C and/or being in gear. Ran a KOER test… no codes. Drove it around some (a few stops and hot starts), no trouble. Nothing.
Win?
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