Miss at roughly 1200 rpm

[DUNE RCR]

Sorry haven't been on in awhile, phone has been giving me issues. Anyways like the title states, I have a random miss at around 1200 rpms. I have new coil packs, plugs and a new throttle position sensor already installed(all motorcraft) and nothing has changed. I cleaned the MAF and IAC with no change either(swapped the MAF from my last truck just cause and still no change). It runs smooth at all other rpms except in that sweet spot. Does anyone have any ideas or ever experience this before?

97 XLT 5.4 4x4

 

[98EXPnSRQ]

Check the EGR valve by putting vacume on the diaphragm. The idle should get rough and when you remove vacume it should run smoothly again.

 

[DUNE RCR]

EGR valve is new along with pipe from manifold. I just rechecked all vacuum line and didn't find anything out of ordinary. I was reading another post where the ports for the EGR get clogged. I will check those next, but my problem isn't at idle, it's during accelerating lightly and right at roughly 1200 (+/-100)rpms. If I jump on the throttle it doesn't miss at all, surprisingly it will spin the tires rather easily(almost 300,000 miles is why I'm surprised).

Also no check engine lights or codes and running my scanner with KOEO/KOER tests don't show anything either.

 

[docraymund]

Do a compression test and/or a leakdown test on all cylinders just to be sure.

 

[Flexpedition]

^^ what he said. have you noticed any cylinder fluid on the ground?

 

[DUNE RCR]

Well that was a lot of fun... Compression tested and all is good, I think... All cylinders are at 170psi except cyl 6 which is at 165 psi. I can't remember the range/pressure that is within spec for these modular engines.

 

[docraymund]

I think you found your problem. The normal range would be the highest reading multiplied by 0.15 then subtracted to the previous value. So 170 X 0.15 = 25.5.
170 - 25.5= 144.5
144.5 would be the lowest acceptable reading.
Your misfire was only evidemt halfway thru midrange because only one cylinder has a slightly low value compared to the other 7 .

 

[DUNE RCR]

I don't see a 5psi difference causing my engine to stumble as hard as it does... It runs smooth in all other rpms too so that's another reason I don't see the slight difference causing any issues. This weekend I will be checking the EGR ports on the intake. If those are clear I will be completely lost on this one.

 

[docraymund]

You forgot to do a cylinder balance test. Try to peg the rpm at 1200 and provide load with a/c on.

 

[CaperCanadian]

I'd also check the coils and plugs, just because they are new doen't mean one is not bad. Pulling everything apart might have got dirt in a fuel injector also. Getting enough fuel to pull away but not enough to sastain an even flow.