got vehicle last year and drove ok. Plugs and wires were replaced soon after purchase. then started to throw a maf sensor code. i was able to diagnose it was the maf sensor and not something else so replaced it and problem was solved. i did replace the map sensor as i broke the old one taking it out. I thought i had an upper manifold coolant leak so had those gaskets replaced and noticed the car would "shudder" while idling. this was something i did not notice, but it could have been there, before. i continued to drive it and when going uphill it seemed to choke a bit so figured it was fuel filter had it replaced seemed to help but then put a can of sea foam in and the shudder and choking came back. no codes at this time. I started some research as i didnt have a good code reader at the time. I will admit i replaced the IAC and IAT sensors just in case.
Needless to say still getting the shudder and last night coming home i had a borrowed code reader that can do live data and had it plugged in. The connection was loose and when i pushed it in the CEL came on blinking and it had code of misfire Cylinder 5. had not had that before but after about 2 minutes the CEL quit blinking and ahsnt come back. The STFT was about -18% and LTFT was -25%. Currently the MAF is 6.18 g/min at idle. the MAP is 9 hg at idle which from what i read seems like it is reading under load. I did a vacuum test and it was within range but the needle did bounce up a little and there was no indication of a clogged cat. I have checked the 5v reference on the maf and it is ok. Did replace the injectors as i got a deal on those. Today i checked resistance in the ignition coils, 1&4 primary 5.8 , 2&5 primary 5.83, 3&6 primary 6.13 and secondary tested at 0.00. I have looked for vacuum leak. Did fuel pressure test got 52 psi. Checked the signal on the TPS and it went up and down smooth while open the throttle. No sign of a bad fuel pressure regulator such as gas coming out but can they be bad without that happening? I did check the plugs to see what they show and would post pictures of them instead of trying to explain. also has short video of the vacuum test i did. Right now it has started to crank and not start very easy until it gets going the on subsequent starts its ok. an y direction tot take now much appreciated. . .

i will try to list things in order as i did them. replaced plugs and wires. had maf sensor code and after determining it was definitely the maf replaced it. replaced map sensor as i broke it. thought i had upper manifold coolant leak so had gaskets replaced. noticed a shudder after this was done. it may have been there before but did notice it. started chugging going up hills so replaced fuel filter and seems to work. put sea foam in chugging back. replaced fuel filter again.then i get a p0172. more research i look for vacuum leak nothing. Fuel pressure test came in at 52 psi. checked plugs but nothing really carboned up but they definitely had some issues. i can post pictures rather than explain if would help. Maf sensor reading 6.18 g/min at idle, MAP 9 hg at idle. did check MAP sensor without breaking it and there is oil under neath it where the PCV sits. STFT bounces around -0.8 to 1.6 and LFTF -10.1-24.0. Voltage on o2 anywhere from 0.950-0.015 this is all Bank 1 as code reader can't get bank 2. Checked TPS signal and got smooth reading when opening and closing throttle.Checked 12v on MAF it is good 5v ref on MAP is good. checked resistance on coils and 1/4 5.8, 2/5 5.83, 3/6 6.13 on primary all secondary readings came in at 0.00. did vacuum test and reading was in green but would bounce higher but not out of green. Listened for and tested for vacuum leaks nothing found, the IAC and IAT have also been replaced. Any direction on what to do next is appreciated.

Merged two threads from same user regarding same issue on same vehicle into one thread.

How many miles are on it today?

How many miles were on it when you purchased it?

When last year did you get it (January? December? June? Etc.?) ?

Where did it live before you got it?

Where does it live now?

How many miles on the engine air filter?

If I had to guess right this minute, I'd guess bad injector.

One other thing you can test is compression on all cylinders.

And yes, please post a picture of the plugs.

thank you for merging these. i did not see that the first one posted.
1. 213k
2. 182k
3.April
4. it had been garaged for a couple of years in iowa
5. outside in iowa
6. not for sure but would say at least 20k
the injectors have been changed since this issue started.
the 2 & 4 plug look like the 1 & 3 not sure why i didn't take a photo of those.
after taking of the ignition coils to test them it seems to start harder like it isn't getting fuel.

not sure why these didnt attach to my reply

So those are new plugs with about 30K on them? They look like OEM, are they?

Were just the upper plenum gaskets replaced, or were the lower intake gaskets as well?

There is no Bank 2 O2 data, because there is no O2 sensor for Bank 2.....

Was Bank 1 sensor 1, removed and checked for contamination, for carbon or coolant? This sensor is the main input for fuel adjustment(Bank 1 sensor 2 is for catalyst efficiency)......

Reading data from this sensor, just shows it'* limits...it'* better to plot the data, looking for a sinusoidal waveform, with plenty of cross counts....sometimes O2 sensors become"lazy" with age......

Have you been losing coolant?

As for the coils, primary R is about 1.0 ohms....secondary R is around 6,000 ohms(6K ohms)......But that is just a static test....obviously, if the readings were off, the coil should be replaced....but a coil can ohm out ok, but still be bad....this can be detected with a scope(how many people have one of these?), or usually with a spark tester.....each secondary terminal should be tested one at a time, by removing a plug wire from the plug and attaching the spark tester and then grounding it, and looking for a healthy spark during cranking......

Also as suggested, a compression test should be done on each cylinder, especially with this mileage....

Thanks for the reply. Good to know these things, and that the air filter has been done somewhat recently.

First: what Tech II said above ^^^^^^

Then: Plug 6 has more deposits on it than the others. Maybe that'* a direction to look? It still doesn't look horrible.

If I were guessing right now, this tastes like bad coil(*), especially with symptoms getting worse when you remove/install them. Regardless of my guess, Tech II has you going the right direction.

the plugs are not oem. i replaced those and the wires after i bought the car. the lower intake gaskets were replaced also. I have not removed the sensor to check it but will do when weather improves. Compression test will also be done depending on the weather in my "garage" as will the spark test. As far as coolant loss, yes there has been some. I am leaning toward an external leak as i can smell it under the hood but evidently it is small enough to not leave a puddle. I am thinking lower hose as it looks like it has some leakage.
Can't say thank you enough as i am losing what sanity i have left trying to figure this out and not just throw parts at it.

Has the upper intake manifold been replaced on this car?

Have the plastic coolant elbows been replaced with metal ones on this car?