2002 Ford Ranger 3.0
Automatic
185K miles

Truck runs great, but here's the problem. If i drive the truck at operating temperature for a bit and park it for 10 or 20 minutes, when I go to restart it, starts up fine but after a few moments from start up the idle drops to 2 or 300 rpm and the engine nearly stalls out, but then quickly kicks up to 1000-1200 rpm and then settles at it's normal 7-800 rpm idle. And then all is normal. Truck cold starts great, idles great, drives as it should; its just the warm start issue i explained above and happens every time i restart a warm/hot engine. There are NO check engine light or codes on my ODB reader.

Things i've fixed/replaced on the truck:
idle air controller
MAF sensor
throttle position sensor
cleaned throttle body
cam shaft synchronizer
piston rings/honed cylinders
crank shaft bearings
replaced driver side cylinder head
new fuel pump/filter
All fluids are fresh
new plugs/wires/coil pack
catalytic converts are good and flow appropriately
solid fuel pressure, 60 PSI, and no PSI change when idle nearly stalls
transmission cooler flushed, new tranny filter/fluid
compression is: 170,165,165,160, 165, 163
no vacuum leaks that I can find

Any thoughts or direction would be great before I take it to the dealership to see if their $200 diagnostic can figure it out.

Thank you

 

,,,oh and the temp sensor was replaced, in case the old one was telling the computer is was colder than it was on warm starts due to going defective.

 

Welcome to the forum

Reads like a computer glitch not a sensor or control issue
Did you use a Motorcraft or Hitachi brand IAC Valve?
If not then put the Factory one back on, 3rd party IAC Valves don't play well with Ford Computers

You can call a Ford dealer and give them your VIN and see if there was a Software upgrade available for your year, might be too old for them to have but they may be able to load new software, I think it runs $150 or so
I think there were a few issue with idle on the early 2000's that didn't show for quite a few years/miles

 

Thanks for the info; i read on a different thread you replied to regarding IAC and i'll be putting the factory one back on.
And i may end up having the dealership try to pin point it. Posting here and a couple other forums is my last ditch effort before I drop $200 for a diagnostic for them to hopefully identify the problem.
It's driving me crazy and i know its likely something i did, missed or changed after all the work i put into the truck because this was not a problem prior to pulling the transmission off to do the oil pan gasket, piston rings, cylinder head job, etc. Before that it just ran rough and had poor compression on bank 2 but didn't specifically rough idle/nearly stall after every warm engine start.

 

Also, RonD, i read some of your posts about fuel trims.
If i watch the live fuel trim data while the rough idle/stall symptom is occurring is there something specific i can watch for to help me ID the problem through the fuel trims or at least help me throw out possible problems?

 

At idle, foot off the gas pedal, computer doesn't adjust fuel trims because it ignores O2 sensors at idle, and at WOT(wide open throttle)
Idle is a richer mix to prevent overheating

Watch RPM, PCM should react instantly to drop in RPM, by opening IAC Valve
Watch spark advance, usually about 22BTDC at idle
Watch MAF sensor, change it to display "grams per second", gr/s should be about 3.0 at 750-800 idle

If these change just BEFORE idle drops then thats the issue, they will all change AFTER idle drops

Try new PCV Valve, these should be changed every 3 or 4 oil changes

 

So checked data...i just have a cheap data scanner...but it tells me the following.

I'm at about 4.2 grams per second at 750 idle. No change in that prior to idle drop/stall or during idle drop/stall. Hopefully that's in the acceptable ball park.

On warm start and prior to idle drop my "Ignition Timing Advance for #1 Cylinder" hops around between 5 degrees and 9 degrees. Then the idle drops like it wants to stall while doing so the degrees are still bouncing everywhere between 5 and 21 degrees. Idle corrects itself on its own and as the engine is stabilizing back to a normal idle the degrees steadily raise from about 8 degrees till they get to about 15/17 degrees where they stay.

 

Here's a 40 second video of warm start with data stream.

 

Sorry first video link was private I believe... This one should open...

 

switch MAF to g/s, 0.61 lb/min is about 4.6 g/s

Seem too high for a 3 liter engine, rule of thumb is 1 g/s per liter of displacement at 1,000rpm
3 liter 3 g/s
4 liter 4 g/s

The computer knows what size engine it is running, i.e. it know a 3 liter engine will pull in 3 liters of air at WOT every 2 RPM, the rest is just math
Gasoline's mix ratio is 14.7 to 1, this is a WEIGHT RATIO
14.7 pounds of air to 1 pound of gasoline

Problem is AIR weight changes quite a bit from sea level to 5,000ft(Denver)
And cold air is much heavier than warmer air, i.e. "hot air rises", its how "hot air balloons" fly, lol

MAF sensor is there to weigh the incoming air, and there is also an air temp sensor
MAF sensor only samples about 10% of the incoming air, so any incorrect MAF data will be times 10 when computer processes it

Computer adds fuel to each cylinder based on the weight of the air at that moment

 

So my computer is dumping more fuel than necessary into the cylinders due to the high air content reading from the MAF causing the bogging down?

If that's the case is the fix to try a different, new MAF sensor or do I have a computer problem?

I also did a reading from cold start this morning. The timing advance was right around 22BTDC after the cold start and as the RPM were settling as you stated was normal so that makes me feel better. However the grams per second from the MAF was still well over 4.0 after cold start and the motor settled at it's normal 750ish rpm.

Thank you again for you info and time.

 

What are Long term fuel trims at(LTFT)?

 

Here's a couple pics of my scanner that I took during idle the other day. I believe this was idle after a warm start and the idle settled back down.

 

 

Short term fuel trims at idle don't mean anything, just FYI, Computer ignores O2 feed back at idle(and WOT)

Long term under 10% is OK

When were the O2 sensors last changed?
O2s are the ONLY sensors that wear out, 100k miles or 12 years, after that they start to run out of the chemical used to generate voltage in the presents of Oxygen, so "go Lean" and waste your money on lower MPG
By the time they actually "fail" you could have bought a new set of O2s 5 times over, lol

 

I couldn't tell if or when they were replaced unfortunately.

I will be switching out the MAF to see if that is the culprit. I re-watched the video and noticed the rough idle starts shortly after start up and the g/ps is 5.6 before AND during the rough idle. Once the computer kicks the RPMs up over 1000 to keep the truck from stalling the g/ps starts to lower along with the idle and settles to 4.2 at 750 RPM; which is still high but maybe acceptable enough to keep a somewhat steady idle.

 

Here's a video of fuel trims at warm start and at the time of idle drop and getting settling back to normal.


2nd link is trims at 2k rpm steady

Feel like I'm back at square one...I unplugged my IAC during idle for giggles and rpm didn't budge. Assuming I have a massive vac leak, but I sprayed carb cleaner everywhere and couldn't find a leak.

Not sure where to go.

 

Remove the air plenum(big hose) from the intake
Start warm engine and unplug IAC Valve
Put you hand over the throttle body opening, may want a glove on, or use a piece of cardboard
Cut off air flow IN so engine idles down, sometimes you can then HEAR the vacuum leak because it will be really sucking hard then
But also a good time to spray

 

I wanted to come back and close this thread out. Ranger's back on the road running great. If I would have listened to the very first thing RonD told me I would have saved a month of time. After a few different IACs, including putting the original back on, I bought a new motorcraft IAC and this fixed my warm start rough idle problem.

Thank you for your help and time. Much appreciated!