Rough Idle In Park/Start

[Cees Klumper]

Is it possible some PO mistakenly used the screw adjustment on the throttle body to set the idle, and now it is set too far open, messing up the idle air/fuel ratio? The ECM would not know that. Best I know, that screw is only to adjust the butterfly valve inside the TB to just-not-bind, i.e. be almost closed, and the ICM and ECM do the idle air to match the fuel.

 

[JerrySab]

Is it possible some PO mistakenly used the screw adjustment on the throttle body to set the idle, and now it is set too far open, messing up the idle air/fuel ratio? The ECM would not know that. Best I know, that screw is only to adjust the butterfly valve inside the TB to just-not-bind, i.e. be almost closed, and the ICM and ECM do the idle air to match the fuel.

Hey Cees!

Definitely been monkeyed with by PO. And then by me. I'll give it a shot!

It actually wasn't screwed in at all when I picked up the truck, and the throttle plate would catch in the body anytime I'd hit the brake. The linkage between TB and cable is also really sloppy/stretched. After I carefully cleaned/degreased the assembly, I could see the remnants of orange paint that - I assume - was the original location of the idle screw. I threaded the screw to that level and then tested voltage at TPS, and found it to be at .92v.. everything I've been able to find says TPS should be as close to 1 is ideal. I just tested vacuum and, to my surprise, needle sits mostly steady at 18.5" hg.

 

[Paulos]

Here's some info if you want to reset the idle screw.

 

[RonD]

Good compression on the engine if you have 18" of vacuum

 

[JerrySab]

Here's some info if you want to reset the idle screw.
View attachment 60172
View attachment 60173

WHOOOAAAA... really fascinating note at the bottom. Thank you for posting these!

 

[JerrySab]

Ok. Quick update.

Did not do the procedure listed above, but did crank down on the throttle plate. Definitely idled lower, ran through warmup cycle mostly fine and settled in about 1k before I cycled through gears. Drove it around the block, ran ok. Felt underpowered, had very different engine note. Turned truck off, grabbed dog to take to park, attempted to turn back on and found it wouldn't stay running. 3 times fired up, 3 times immediate stall after turning over. Opened screw a little more (didnt test voltage at TPS) and found truck would stay running. Ran fine, ish. not sure if computer needs 100 miles or so to digest the change? Stinks like eggs. Think new injectors are absolute hoses. Also, I fear I have a non-motorcraft IAC and it might be throwing off the mix. MAP sensor looks non-rotorcraft as well.

Bye money.

 

[Cees Klumper]

SOmething sounds definitely off if adjusting that plate stop screw has such a major impact, that should not be. Sounds like the IAC is plugged up/not functioning or what have you. Also, your throttle position sensor may be off/broken?
I just applied the above procedure and it did work as described. I had too high an idle with the IAC disconnected and after running at 2,000 rpm for two minutes. Adjusted down, stopped engine, turned back on, adjusted just a bit more. Stopped engine, reconnected the plug to the IAC and now runs just like it should according to the emissions decal (around 800 in neutral, almost the same in drive). My TPS gives off .98 volt at idle, 4.7 at full throttle (backprobing the middle pin, negative to ground), so that seems good also.
I've had an intermittent stall/stumble at idle I can't seem to get fixed. Replaced the IAC with a Motorcraft one, replaced the ignition coil, O2 sensor, checked for vacuum leaks, replaced the computer and mass airflow sensor (CA versions, I have spares for this type of situation) but nothing seems to be 'it' entirely. Main culprit that did help so far was a bad booster one-way vacuum valve (it was letting air through both directions) and now it just stumbles every once in a while, it doesn't completely stall anymore like it was all of a sudden doing. When driving/accelerating, there is 0 issue - it's only at idle. Maybe it's the fuel pressure regulator that's having an intermittent issue. To be continued.

 

[JerrySab]

Cees, similar problems amigo!

You could always try and replace fuel filters? I found that 2 of my injectors were faulty, and replaced them with injectors that flow way more fuel than their rating implies. The replacements haven't arrived yet. Really eager to get them plugged in.

 

[Cees Klumper]

Yeah, I also wondered about the fuel filters (and the fuel pump) but those were all replaced by the guy who sold the truck to me, still look new, and I would expect that the issue would be worse under load, when more fuel is required, than at idle but there's no issue at all when driving/accellerating.
When I put in a set of rebuilt injectors, that was the single biggest improvement I made to the engine. About 18 months ago that was, I can't imagine one or more going bad already, and, again, I would think they then malfunction also under load, not only at idle. Perhaps it's a wiring thing or the fuel pressure regulator, I will just keep at it. Good luck with solving your issue!

 

[JerrySab]

Thank you all again for the great advice!

Turns out, the replacement injectors were very much the problem. BEWARE 4-hole non OE low flow injectors that flood engine with fuel mist, rather than the focused spray from single pintle. What a pain in the ass.

Went with remanufactured injectors and they're awesome. So glad to have my truck back, and running very strong. No more mothball smell either, so grateful I didn't fry the cat.

 

[bobbywalter]

Yeah...they were off for the whole range then ...


Brand new out if the box junk....


...you will have that.

This is why access to wideband O2 is so nice.

 

[JerrySab]

@bobbywalter you were pretty close with your guess of lean reality despite rich code. I rigged up an injector tester with a small 12v battery, some hose & clamps and a can of carb cleaner. The spray pattern between OE and the new 'replacements' were very different. New off brand injectors creating a thick mist of gas that would never reach spark. Leading to weak combustion, but TONS of unburned fuel going past o2 sensor. So absolutely lean performance, rich code, and nearly burned out cat.

 

[Cees Klumper]

Congratulations, always nice when something gets sorted like that. 'Bad out of the box' is only too familiar to me. I also replaced my injectors with remanufactured (original Bosch) ones and they dramatically improved my truck's performance. About 18 months ago as I recall, and only $60 for 6 of them (from CS Performance, they advertise on Ebay), delivered. I believe the price went up a bit, but still a bargain compared to what new good quality injectors can cost.

 

[Cees Klumper]

Actually my idling problem seems solved, after I applied the adjustment procedure for the throttle body 'butterfly valve adjustment crew'. Nice and even idle, no more faltering or almost-stalling. I now have a different problem (it's always something it seems) with the truck just not starting at all on colder mornings. (Relative term here in Southern California). Twice now this week, it refused to start (starter cranking and cranking, but no start) and I could tell it was the fuel pump just not turning on as it should, when turning the key to 'ignition' for those couple seconds to build pressure - nothing at all. After it warms up in ambient temperature, it starts no problem and will restart all day long. I suspect one of the relays, under the power distribution box (either for the EEC/ECU or the fuel pump relay) OR the ignition module - although I replaced all three not that long ago except the fuel pump relay - as that looked almost new when I first took a look at it. 'Bad almost out of the box' may apply here. Off to order me a fuel pump relay and ignition module first, see if that's it. (Previous owner also replaced the fuel pump inside the tank - hoping that's not gone bad within 2 years so I don't have to drop the tank etc).

 

[bobbywalter]

May be just crappy corroded connection.