2.9 engine miss ideas??

[Tuner710]

Trying to isolate reason for miss / shudder with high idle in 86 2.9 engine. Fuel pressure solid 30. Base timing set to 10 deg btdc. Cap and rotor new and clean. Plugs new and clean. Wires new. Spark present at all 6 cylinders tested with spark tester light. Vacuum steady at 25 or so. IAC and TPS both new, wires and connections good. Checked all vacuum lines I can find and seem ok.
When setting timing, with the spout connector out, the mark is rock solid at the pointer, as soon as I plug in the spout it advances to around 20, fluctuating and the idle jumps up.
I tested for a vacuum leak with spray all around the intake manifold and got no change in the idle.
So, having walked away from it in frustration, I thought it must be time to ask for help from the best and brightest here. Any input is greatly appreciated!

 

[RonD]

Well you need correct spark, correct fuel/air and high compression, for a smooth idle.

Sounds like spark is OK, but older distributors can get a wobble at low speed, which can cause missing that goes away at higher shaft speed.

Fuel/air mix can be a vacuum leak or a dirty injector, at idle the fuel drips instead of sprays so doesn't get a good mix, some Seafoam in the tank can help clean things up, but if you put in Name Brand gas it usually has injector cleaner in it.

Lower compression in a cylinder can cause it to miss fire at idle.

New IAC....new doesn't mean "it works" any more, new means "never tested" now a days, lol, so I would check it just to be sure.

 

[Tuner710]

Ron - thanks for the ideas. I think I'm going to pull the cap and rotor, and press on the shaft to see if I get any play.
In the back of my mind I have been thinking about dirty injectors, was hoping to fix the problem by now without tearing into the intake. May have to though, it could be injector problem because there was very dirty gas in the fuel filter when I changed it. I will try some fuel system cleaner and better gas first.
Again, appreciate the help!

 

[RonD]

If it is a steady miss, almost a lope, then it is one cylinder misfiring, which could be an injector or spark plug and wire.
You can ID a misfiring cylinder by unplugging one spark plug wire at a time, firing cylinders with no spark will really show up, misfiring cylinder will barely change the way the engine is running with no spark.

If you can ID any cylinder then swap plugs, wires, and injectors with work cylinders and see if the problem follows the injector or plug

 

[a31ford]

Question,

Does it "stumble/moterboat" at highway speeds, AND, does the Tach bounce around, (generally HIGHER) than stated RPM when it's doing it ???

IF so, replace the TPS

TPS will cause a false read, and make IAC do a higher than regular idle even when warm.

I fought with this issue for 3 months.

fuel sock IN the tank, (Volume, NOT pressure),

KOEO and see if you are getting a 22, 23, & 53 in KAM (second half of motor off test).
As well, DO A KOER, and note the high an low RPM's after the goose test.

make sure, you are getting a P-brake light on the dash (when set) for the test run, IF not, the ECM test will be worthless.

Greg

 

[Tuner710]

I pulled plug wires while engine was running, after I had tested for spark with the light, to see if a plug misfiring or fuel problem, each time the engine ran noticeably rougher.
I have driven it up to 45 mph, and it seemed to actually run somewhat smoother with higher rpm. Did not notice the tach bouncing around, I think it would have stood out if it had. Greg, what would that indicate?

 

[Tuner710]

I read my original post and see I forgot to mention that I ran a comprssion test. Best I could by holding the guage into the spark plug hole with the cone, the adapters available werent long enough to reach the threads. Waem engine, all plugs out. I got 90 to 91 in five and one 80.

 

[RonD]

Compression should be around 170+ but holding a pressure gauge against that kind of pressure isn't possible, so all between 80-91 means there should be no problem with 1 bad cylinder.

Another thing to try would be to remove and plug one vacuum line at a time, may be find a leak that wasn't found by spraying around the intake.

 

[a31ford]

I know the TFI can be an issue, BUT I'm assuming that this is when it's cold (start-up).

Get the codes, and we will see...

LOL, my truck is a 'work truck as well"....

 

[RustedRanger]

On mine I got a bad rotor for the dist the first time....it fit way loose. Something easy you can try is to clean the back of the throttle blade with cleaner on a rag,even after spraying the throttle body out there was a huge amount of black crud on mine and that can cause weird problems. If you didn't already try replacing the PCV valve with a good brand name one.

 

[Racenut]

Sounds like mine is doing the same. I pulled the injectors. They were clogged!!!! I orderd some remaned ones. Will b here in a few days. So I hope that will fix mine. BUT, could b something for u to look into. Mine had sat up for a year.

 

[a31ford]

Tuner.... did you get the codes out of the computer ???

Greg

 

[Tuner710]

Sorry hadnt checked back, work happened...be a few days before I can get after it, which sucks cause I need it running, it has gas in it lol. Anyway, I did a KOEO and got a code for TPS voltage problem. I checked wires and connections, ended up replacing the TPS even though I had a hunch wasn't likely. Didn't change.
Going to make a pigtail at work so I can thread into plug hole for a good compression test, and then thinking pulling the bed again to see what the strainer looks like. Next move would be injectors, they aint cheap if I need one or more. Got noid light as a start anyway.
Thanks again to all for the input, it really helps get me guided in a direction. I know I'm skipping around somewhat in troubleshooting, keep hoping to stumble upon the easy fix, right?

 

[Tuner710]

Looking back over posts, and realized I hadnt followed up on this problem.
After a lot of quality time spent under the hood, I bit the bullet and tore into the injectors. Pulling connectors one at a time while running wasn't totally conclusive, because it was running real crappy anyway.
Anyway, clogged injectors and at least 1 that was stuck, replaced with Bosch 19 pounders, and rig is purring now!
The moral I guess is, even though its a lot of work to go after the time consuming pain in the ass possible problem area, it has to take less time than all the rest of the troubleshooting combined lol

 

[RonD]

Thanks for the follow up post and the fix for the problem