Motor Stumbles under Load

[flint54]

I have a young (90K miles) mostly stock 4.0 in a 1992 Comanche that was in excellent condition until
I started making it "better". To gain a bit more power I've done the following: 63mm throttle body,
K&N FIPK air intake, removed mechanical fan, new 20lb 4 hole injectors, Hesco AFPR (set at 32psi),
3 angle valve job, 0.020" shaved from head, minor porting of head, and I installed the 99 intake
manifold. That's when the trouble began. Now, the motor misses around 2500 rpm, particularly
under load. It also will not idle worth beans, and if I let off the throttle suddenly it stalls. I can rev
it to redline repeatedly in the driveway, no issues. Manifold vacuum measures steady 18 at idle, and
behaves correctly when I blip the throttle (spike in each direction and return to steady). Compression
is strong, 165-70 in all cylinders. In attempt to solve this, I have done the following:
New Champion plugs gapped to 0.035". New OEM distributor (had a bound up squeaking shaft in the
old one). New plug wires (Belden), New OEM rotor and cap. New OEM coil. I've cleaned the ECU
connector pins, and verified good path from ECU to coil. I've looked for intake leaks with propane
and cannot find any. I've verified good fuel pressure and fuel flow. The miss is severe, and is as
if the coil gets momentarily unplugged. The tach also blips when the miss happens. I've also replaced
the IAC, and made sure the TB port is clean. My friends over on the Comanche forum are as
puzzled as I am. I've run it like this for almost 500 miles, and the plugs are equally tan, no sign of
too lean or too rich on plugs. I will be immensely grateful if anyone here can help me solve this
vexing challenge. Swapping the manifold was the last change I made, and motor ran great with all
the other changes. It was torqued carefully and rechecked several times. Thanks!!

[92tank]

have you tryed running the stock injectors again. if its mostly stock you should not need any bigger injectors. or you just have a bad or sticky injector. my stroker did the same thing with a bad injector. another thing to consider is the thottle body butterfly, is it adusted properly?

[flint54]

Well, I'm pretty sure these injectors are same, or very close to, stock spec, and they
were flow tested. Tell me more about butterfly adjustment. All I know is the tiny
setscrew that sets the position so the edge doesn't gouge the bore and so the throttle
opens smoothly without sticking. Is there more? The stumble seems to be electrical,
just like flipping a switch. Would a sticking injector misbehave only with load and
not other times?

[SilverXJ]

Have you checked the TPS?

From Dino's page on fuel injectors:
'91-'93, ........33007127, .........Brown, ..........39psi, .................21.0lb/hr

I don't know what injectors you have besides 20# (don't know @ what pressure or part number) but if you reduced the fuel pressure as well as installing smaller injectors you might be a bit lean. Maybe try upping the fuel pressure a bit and see what happens.

But I would check that TPS first.

And do not touch the Butterfly set screw. Its not for idle adjustment. It is just to keep the butterfly from hitting the bore of the throttle body. Where ever you got your throttle body from should have adjusted it correctly.

[Cheromaniac]

The miss is severe, and is as if the coil gets momentarily unplugged. The tach also blips when the miss happens.

That definitely isn't a TPS problem. A bad TPS will cause some hesitancy when you change throttle position and cause a surging idle, but stalling and missing are symptoms of something else. Combined with the bouncing tach I suspect the problem might be a failing CPS.

[flint54]

Regarding the injectors, I was using a round number of 20. They calculate out to closer to 19lb/hr, based
on the flow sheet showing an average of 209cc/min at 43.5 psi. My original fuel pressure (before AFPR) was
30 (with vacuum), 38 (vacuum removed). I bumped it up a few points to 34/42, and observed plug fouling
so backed it down to 32/40 where it remains, and plugs look just right. These injectors are equivalent to
stock.

Regarding CPS, I failed to mention in my first post that I took advantage of the extra room while the manifolds
were off to install a new CPS.

Regarding the butterfly set screw, I understand that it does not, and cannot, control the idle. When I installed
the new 63mm TB, I was experiencing a badly sticking throttle, closed position only, and surmised that manifold
vacuum, combined with the awkward geometry of the cable-to-linkage connection was the cause. Opening the
butterfly slightly using that setscrew was the solution. It was definitley not adjusted correctly from the supplier.

The stumble happens as if I switched off the coil for maybe a 1/4 second, and first happens around 2500 rpm
if I'm moving and encounter a hill or step on the throttle (increased load). It will happen worse near 4000 rpm.
I've replaced everything that I can think of electrical. Still very puzzled.

[MoFo]

You said the last thing you did was install the 99+ intake manifold, as if that could have been the cause, yet you say it "sounds like" an ignition problem. Now you add that you replaced the CkPS when you installed the new intake. I agree with Dino. My suspicion is that you put in a defective CkPS when you had installed the new intake. What was the source and brand? If it was an Airtex, go to the stealership and get a real sensor.

[flint54]

To get the chronology exactly correct, the last thing I did was replace the distributor.
The original distributor was chirping loudly. When I pulled it out, the shaft was so
bound up I could not turn it by hand. Since the dist only has one correct position,
I ruled out that as a cause of the problem. The stumbling problem commenced
after replacing the manifold, even though it doesn't seem that the manifold could
cause what "feels", not "sounds", like an ignition problem. Since the motor ran fine
before this project,I conclude that the original CPS was not defective. I replaced it
because it was 20 years old and I had the opportunity, not because it was flawed.
The replacement was Crown brand, from Morris 4x4 in south FL, rather than OEM. Have
no idea if those are good or not. It would be simple enough to reinstall my original OEM CPS
and see if that makes any difference, since it was known good just prior to pulling the
manifold.

[flint54]

Update; I have swapped back in the OEM CPS that was known good just before the manifold
exchange and it made no difference to the problem. Still stumbling under load, and also
now stumbling (around 4000 rpm) if I rev it standing still. Really ratty. What's next??

[Cheromaniac]

Check for a vacuum leak between the intake manifold and head.

[racing89]

Your 99- head equipted with an egr? If so, unhook temperarily, as a defective one can cause your exact symptoms. Also check for fuel contamination and a cracked plug.

[herbiehind]

x2 on vac leak . thought fuel pressure was 49 on these?

[flint54]

Regarding vacuum, back in my original post I said: "I've looked for intake leaks with propane and cannot find any."
I suppose it is possible to have a leak that I cannot detect, but flammable gas aimed
all around the mating surfaces does not raise the RPM.

Regarding the head, it is the original 92 HO head, not a 99 head, and there is no EGR.

Regarding the fuel pressure, it is currently set per FSM requirement for stock. I too thought
I needed more fuel, that's why I added the AFPR, but when I increased the pressure the plugs
fouled quickly and badly.

Plugs are brand new, I've pulled them several times to check color, and none are cracked.

I use good fuel (Chevron), and have good filter. I drive enough that fuel does not linger in the tank.

The tank was recently out (for cleanup of the region) and it was very clean inside. No crud.