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Rough Idle In Park/Start


JerrySab

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Hi friends!

Playing whackamole with idle problems on my '88 XLT resto project. While I still have many knowledge gaps, I definitely appreciate everybody's wisdom here, as that has gotten me very far on this project.

TLDR question:
Can somebody explain to me why the truck is a shitshow in park and totally fine in gear?

Expanded question:
- 1988 2.9l XLT (Little Gold) has new cylinder heads, plugs, fuel pumps, IAC, TPS, MAP, distributor & wires, electric fan w temp switch, thermo, CTS, ICM, O2 sensor.
- After replacing inline fuel pump, truck always started first time and had good fuel pressure, but would take looooooong time to warm up and idle down to 850 rpm - if it ever got there. Once it would go under 1k, it would start missing and be generally fussy/unpleasant. Driving in gear was ok, def weak. MPG ok. Compression ok. OBD1 reader would show 11 (no codes/all good).
- Injectors, plugs, 02 sensor were last items to be upgraded, all at once. 2 injectors had bad o-rings, with one pintle cap long gone. When in D, idle is smooth. Power is great. Startup is a mess. Comes up to temp much quicker, idle falls from 2k, 1.5k, then gets to about 1.1k/1k, and then it stumbles and either stalls, or saves itself, bumps up to 1200 or so, then flirts with 1k rpm before quitting. Starts back up fine, gets into gear fine and is mostly problem free. The occasional misfire/stumble/wobble at a stop light but nothing serious. Doesn't stall. When put back in park, RPM goes all over the place. many misfire/stumbles. searching for right RPM range. it kinda smells like mothballs? I have not pulled codes or done compression test since replacing plugs & injectors.

MY FEARS:
- Did I crack a head while trying to dial in temp switch for electric fan? It was set too high, and saw coolant temp gauge get to the top of threshold before fan kicked in. N in normal. not beyond. Coolant level is fine/not falling. Oil level is fine/not falling. Don't see milkshake in oil, don't see oil in radiator or overflow. Tried glove test and didn't see any inflation/bubbles. have not pulled plugs to do compression test yet.
- I may have killed brake booster check valve? Pumping brake will elevate RPM in park. didnt notice this before replacing injectors. I may have inadvertently yanked too hard on it trying to get fuel rail out.
- Is intake gasket bad? Would a vacuum leak cause these kinds of problems in Park but not in drive?
- Could ICM be bad? I have a replacement from a fat finger mistake, but haven't tested the one in the truck. Wouldn't that affect higher RPM range and not lower if it were bad?

To anyone who has wisdom or thoughts, would love to hear it. Thanks everybody!
 


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RonD

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With a 1988 I would certainly have a look inside the computer and at its 60-pin connector and wires

Does engine run better cold than when warm, or visa versa?

After engine is warmed up, and idling, unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or engine may even stall, either is good, it means no vacuum leaks

Yes, brake booster is leaking from your description

PCV Valve should be closed at idle, it can stick and cause a vacuum leak, plug the PCV valve hose and see if idle improves

Warmed up engine, hook a timing light up and test timing at 1,000rpm and then at idle, not looking for a "degree" looking for missing spark pulses causing the misfires, so should still have the steady rhythm of a spark every 2nd rotation
Test #1 but also test a few others for the steady pulse

Its possible its a compression issue, best thing is to just do a compression test
Cold engine
All spark plugs removed
Test each cylinder with throttle wide open
Expected is 150-160psi with a 2.9l, but this is dependent on compression gauge and battery condition(cranking speed)
What you are really after is an average PSI and if any cylinders are 10-15% lower than that average

Compression is related to the speed of the piston as it travels up with both valves closed
Metal valves on metal valve seats and metal piston rings on metal cylinder walls are NOT air tight, lol
So the faster the piston is moving the less time there is for the air to leak out
So a low compression misfire at idle can go away as RPMs(piston speed) goes up
 
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JerrySab

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RonD! The Legend!

Thanks for this. Just pulled codes, got 42 - rich condition. I'm thinking the new injectors might be flowing too much fuel. Not sure if that would account for all of the issues? But in my (limited) understanding, it does make some sense with the drive/idle in D quality - which is high - vs the idle in park/startup quality, which is very low.
 

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:)
Well the 2.9l computer is pre-set for 14lbs injectors, so if you put in larger injectors you would get a Rich code
 

JerrySab

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They were indeed rated for 14#, but after the fact, I did find several complaints buried in the reviews of these particular injectors that they were flooding combustion chambers. They were reeeaaaally cheap. I'll check wiring tomorrow to make sure each is getting power.

Ron are injectors able to restrict amount of fuel per spray? Or is it the same amount of fuel with each spray, and frequency of spraying increases/decreases per engine load?
 

bobbywalter

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Your injectors are batch fired.

They are duty cycled for regulation of amount they spray.

So 3 are fired at a time. Pins 58/59

So cold start and idle can definitely be affected by improper sizing or function.

But a propped open or hanging throttle body can do that to...
 

JerrySab

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Thank you Bobby Walter, understood!
 

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Computer calculates the open time for an injector
A 1988 2.9l Computer "knows" a 14lb injector with 35psi fuel pressure will allow XX pounds of fuel to flow thru it if its opened for say 100ms(milliseconds)

If injector flow(14lbs an hour) or fuel pressure is changed then computer's calculation will be incorrect

Air/fuel mix ratio for gasoline engine's is 14.7 to 1, this is a weight ratio
14.7 pounds of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline

This is why injectors are rated in Pounds per hour
If more gasoline is flowing out because injector is 19lbs then Rich code

There is no fuel pressure sensor, if fuel pressure was say 60psi then more fuel would flow out during the 100ms than at 35psi, so Rich code

"New" means "never EVER tested" now-a-days, so new injectors could be the issue, especially if they have gotten bad reviews/reports
 

JerrySab

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Fingers crossed, I hate trouble shooting after ‘fixing’.

this is such great information, thank you so much for sharing your knowledge!

I still don’t understand why it runs so poorly in park and seems to function so well in drive though. Is EEC instructing lower fuel rate in P, and perhaps over-delivering injectors are throwing it off? And then higher fuel rate is expected in drive/driving conditions?

this is purely for my own understanding. And an odd tic I’ve noticed with this truck since acquiring last year.
 

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At Idle or when engine is cold computer doesn't use O2 sensors, or at WOT(wide open throttle)
Computer runs a pre-set fuel mix during these times, based on RPMs, throttle position and engine temp
Computer also has an engine "load" value, in 1988 2.9l that's from the MAP sensor, so in gear, even while stopped there is an added "load" on the engine from torque converter, so computer will adjust fuel mix for that "load"

If injectors were the issue, big if, then when driving the computer would be using O2 sensor feedback so could readjust injector open time on the fly if there was an issue with injectors flow rate, which it won't do at idle
Try WOT, in a safe area, lol, and see if engine runs rough with no O2 sensor feedback
 

JerrySab

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Would unplugging o2 sensor accomplish same thing?
 

bobbywalter

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That will limp it eventually...


Depending on if it's an auto or manual ..there are different feed strategies. So bad ECM is possible..
Or mismatch of ECM ECT..

If there is a cam change or the injectors are fat at idle from low or excessive fuel psi...or the throttle is propped open...to the point the iac values out...you will have idle surge.

In drive the load smooths the map.

Rich codes can be tricky on obd1. If it is a rich limit...usually that means it is actually lean...thus the mothball smell...it has fattened up to it's max table parameter and still reads lean....or thinks it's lean..this can surge idle as well ..and ping under acceleration...if it's vacuum.


Hard to predict and trouble shoot these old critters.
 

bobbywalter

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Make sure the o2 heater circuit has power as well.
 

RonD

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+1 ^^^^

Unplugged sensor is different that unused sensor, computers have loads of "if/then" in the software
People will often unplug a sensor and engine runs better so "it must be that sensor was bad" but it does always work that way, as the computer changes the if/then pathways when there is no sensor vs just ignoring a working sensor

Really good reading here on the older Ford systems: http://www.fuelinjectedford.com/
 

JerrySab

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GUYS YOU'RE BOTH LEGENDS THANK YOU!!


That will limp it eventually...


Depending on if it's an auto or manual ..there are different feed strategies. So bad ECM is possible..
Or mismatch of ECM ECT..

If there is a cam change or the injectors are fat at idle from low or excessive fuel psi...or the throttle is propped open...to the point the iac values out...you will have idle surge.

In drive the load smooths the map.

Rich codes can be tricky on obd1. If it is a rich limit...usually that means it is actually lean...thus the mothball smell...it has fattened up to it's max table parameter and still reads lean....or thinks it's lean..this can surge idle as well ..and ping under acceleration...if it's vacuum.


Hard to predict and trouble shoot these old critters.
BOY I'LL SAY.

But appreciate all this wisdom. SOoooo you're saying the mothball smell would be indicative of lean condition? I assumed that was the smell of unburned fuel, but it doesn't necessarily smell like fuel. Everything you described as vacuum leak lines up. I do believe the check valve attached to the brake booster is bad. Airflow moves 1 direction there, correct? As in, I shouldn't be able to pump air back into the system by pressing brake pedal.
 


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