As far as i know this has been going on before i did all my work to it over the weekend but i'm not for sure. Anyway, i just replaced plugs and wires, Idle air control, DPFE sensor, put a can of injector cleaner in it, changed lash adjusters, oil, air and fuel filter, cleaned the throttle body and MAF, and reset the computer. When we started it up, it idled good, didn't skip a beat but after giving it some gas and letting off, it kills itself, or almost does until it comes back up to normal RPM, and even then it shakes and carries on. Also when its just above idle, it acts like its missing, but give it more gas and it runs fine. Driving down the road it does fine, and going up hills i cant feel it missing. We are now searching for vacuum leaks but aren't having too much luck. I'm at my wits end of trying to figure this one out. Sorry for my horrible explanation.

 

After engine is warmed up and idling, un-plug IAC Valves 2 wire connector, RPMs should drop to 500 or so or engine may even stall, either is GOOD, it means no vacuum leak

If idle doesn't drop then you have a leak

Hesitation off idle could be spark or fuel, the TPS(throttle position sensor) gives the computer a "heads up" to advance spark and add more fuel instantly vs the MAF or Cam sensor which would have a delay as RPMs and airflow increases
Make sure it got plugged back in and connections are good

 

Thank you for the reply!
I unplugged the IAC when the engine was both warm and cold. When it was warm it didn't drop, and when it was cold it did.
We looked for a vacuum leak but couldn't find one other than a tiny hose for the door vent between defrost, front, and floor that goes to the ball on the exhaust side of the engine bay.

Just a side note, (ill make a list of what happened because i'm terrible at explaining things.) we were experimenting with it again.
1. started and let idle, runs fine, no shimmering.
2. revved up engine from throttle body and let off quickly.
3. engine almost dies (or dies) and shakes violently as RPM,s raise back to normal.
4. Miss is present for around a minute after near-stall, and engine is shaking.
5. engine returns to normal with no visible shaking.
The lower RPM its revved to, the less it shakes after you let off.

 

IAC test is only valid with warm engine, cold engine is in "Choke Mode" so has a preset high idle and idle will always drop if you unplug IAC, but it does mean IAC valve is working

Have you cleaned the IAC Valve?

Any 4cyl engine will shake like crazy if RPMs get too low, say under 650rpms, this is because they only fire a cylinder every 180deg of crank rotation, this makes them vibrate at lower RPMs
Most 4cyls idle at minimum 750-800

With warm engine idling and IAC Valve unplugged, remove vacuum lines, one at a time, from the intake manifold, and cover its intake port with your finger, if RPMs drop to 500 or engine stalls then that hose has the leak
Start with PCV Valve hose, a failing PCV Valve can present as a vacuum leak

 

We cleaned the IAC valve before we put it in.

Will check for a vacuum leak soon, and by intake port do you mean on the hose or the barb it connects to on the engine.

We hooked up a fuel pressure gauge up to it as well and it was good 60-65 for both engine off key on, and idling.

Could it be an injector problem? I'm not understanding is that its intermittent. On my way to school it never skipped a beat. On my way back it almost died at the stop light. and it runs perfect just idling but as soon as you rev it up and it comes back to idle it starts shuddering. then it goes back to normal after about 30 seconds.

 

We looked again for a vacuum leak and believe to have found one around the PCV valve. replacing valve and hose. can the canister that the PCV plugs into be replaced without pulling the engine?

Could it also be where the new lifters are holding the valves open? when its idling normally the vacuum gauge its not fluctuating, but when its shaking, the gauge fluctuates rapidly.

 

I'm subbing to this because my 2.3L does the same thing. I thought it was plugs and wires so I replaced with a fresh set of Motorcraft. After replacement I fired up the engine, it idled smoothly, then I revved the engine. As soon as the RPM's came back down the engine sputtered and shut off. It did it to me this morning as well.

It was colder this morning than it has been since I got the truck (1 month ago). I started the truck, it idled fine. I left my driveway and went down the street to a stop sign (about 300 yards away) I stayed in second gear so the RPM was kind of high ~around 3,500 or so. When I came to a stop at the stop sign the engine stalled.

@RonD Should I be checking the IAC and looking for vacuum leaks for this issue as well?

Thanks!

 

Yes, I would check those

IAC Valve can only raise RPMs to 2,500 or so, not 3,500, so yes probably some kind of vacuum leak going on

 

Sorry for the confusion. I meant I was driving the truck at 3,500 rpm since it's a manual. The stop sign is so close that I didnt feel like shifting into 3rd to drop the rpm.

Still think there's a possible vacuum leak?

 

Glad im not the only one with this problem. let me know if you find anything. does your truck still do it after about 20 minutes of running? mine gets alot better after it has been running a while. I have been driving my truck 60 miles a day back and forth to college. it used to only do it coming home in the afternoon but now its doing it almost anytime i start the truck. I haven't found any vacuum leaks and cant hear any. I've replaced a bunch of stuff related to it and even bumped the idle screw up so its idling at 900-1000 RPM. No difference. Ron, do you think an advanced obd2 scanner might be able to tell me more, something with live data? also, when it isn't doing it, it runs like a sewing machine, if it was a vacuum leak or lifters holding the valves open, it would do it all the time right?

 

yeah mine seems to run fine as soon as the engine starts to come up to operating temperature. it just acts up when its cold.

 

Mine does it when its cold and it also does it when its warm, but after about 20 minutes of driving its better. it did get worse when we put plugs, wires, and lifters in, cause it didn't do it before.

 

I put my hand over the exhaust when it was doing it and it sucked my hand towards it , then blew it away. It does get better after it warms up. every light i stopped at it got a little better. i was reading another post about exhaust and was wondering if it could be a head gasket issue? If it was doing it all the time or was missing under load, it would be easier to diagnose, but since its intermittent i'm lost.

 

it didnt run good both at school or at home today. Im stumped. Like ive said before it cant be anything mechanical because at times it runs like a sewing machine. I would think that if it was mechanical, it would run like crap all the time. It has to be a sensor, computer problem. i hope.

EDIT: also i was messing around in the parking lot today and noticed it surging ever so slightly in midrange first gear. holding steady at 5-10 mph. also anytime the engine revs up just above idle it shakes and carries on. after a certain RPM above that it goes back to being a sewing machine.

 

heres a video of me holding a towel to the exhaust. phone cut out but some of it is still there. this was this afternoon when i got home. at this time it usually runs good. today it didnt. sitting in the school parking lot was alot worse.

 

You have a misfire

Unplug one coil pack and start the engine, see how it idles

Shut it off and switch coil packs, to see which one has the worst misfire

Inspect that coil pack for cracks, replace if any are found
Pull out spark plug on that side to have a look at the tips

Its great to swap out sensors, but in reality sensors are often swapped but rarely fail, something like 90% of swapped sensors were working fine

Run some Seafoam or similar injector cleaner in the gas tank
Dirty injectors can cause al sorts of come and go issues

 

OK thank you! I noticed on one of my coil packs the plug is loose because a clip is broke. don't know if that's the problem or not but i will investigate tomorrow. is there a way i can isolate it to a certain cylinder since there is two spark plugs each?
Last week i ran a tank of injector cleaner through and right after that a bottle of Seafoam. how could i tell if one of my injectors are clogged or not working right?

 

Run the engine with just ONE coil pack connected, drive it that way, then check the codes, if there is a steady or intermittent misfire the code number will tell you which cylinder was misfiring
Then repeat on the other coil pack

If same misfire code comes up then could be injector issue on that cylinder or low compression on that cylinder

 

ok. so if im undersatnding correctly, unplug a coil pack, drive it until CEL comes on, check code, switch coil packs, drive until CEL comes on, check code. if codes are the same then its something other than spark? Is there different codes for each cylinder im assuming?

 

Yes, P0301 = misfire cylinder #1
P0302 = cylinder #2
ect....

P0316 is random misfires on startup, so no specific cylinder has a steady misfire, it not listed in all lists but is a valid code
Can also mean cam sensor is not synced to crank sensor, crank sensor is main sensor engine can't start without it, cam sensor is used once engine starts for sequential fuel injection timing

You will also get a code for the unplugged coil pack

Full code list is here: https://therangerstation.com/tech_li...II_codes.shtml

 

ok, will do. Is this something i can do just driving around town, id rather not do it driving to school down the interstate.

 

Yes, anytime is fine, just need to warm up the engine, so at least a 10min drive

 

ok. I got a video showing exactly what it does, better than the last one. when i get it sent to my laptop ill post it.

 

Heres the video. when i rev the engine its hard to notice on camera but the engine is shaking at idle and above idle.

 

Hi Nathan, I subbed to this thread because it sounds like yours has the exact same problem as mine.

Same symptoms, and I even have the same broken vacuum line as you... The grey one, in front of the heater core, that goes to the Max AC duct.

On a side note, to check the other end of that vacuum line at the duct, it's easy to open the glove box all the way. Just release the tabs on each side, and the door drops wide open to access everything inside there. On mine, everything looked ok from what I could see/touch in there.

On mine, the CEL doesn't work, so I went to Autozone to get the codes read. P0102 came up and they suggested replacing the MAF. I held off on buying it, because their suggestions don't always work, and the part was close to $100.. I have some MAF cleaner, so I cleaned it, but that didn't solve the problem. I have to learn how to test that MAF before I try replacing it.

I've seen a lot of suggestions about cleaning/replacing the IAC too, but I didn't go after that one yet.

I'm still hoping to find a simple vacuum leak somewhere. I used to have a '98 F-150 that had trouble idling, and I wasted a lot of money on things that didn't fix it. Eventually, I learned about a vacuum line that's really hard to find... It can't be seen... You just have to know where it is, reach in there and feel around for it. As soon as I touched it, I knew I found the problem. Once I found it, it was a $3 fix... But it sure was hard to find!

So, good luck with yours Nathan, and I'll post up if I find anything on mine too.