Jeep Strokers

Hi all, first post here hoping someone can give me a few clues - please!

I completed my first stroker installation into my 1984 CJ-7 w/- NV3550 a couple of weeks ago - here are some basic details:

Donors - OBD1 4.0 came from a 1994 XJ Limited AW4 Auto (300,000km odo) crank came from 1984 4CW 258
Full rebuild - 0.30" over, decked block, Comp 68-232-4, dished a set of H802CP pistons for 9.7CR, ported the 7120 head and matched it to manifold, 63mm TB, K&N FIPK (4.0 YJ), Banks Torque Tubes (4.0 YJ), 24lb ford injectors. Nothing unusual about any of it. I'm loving it in the CJ - it is just great, aside from one niggly little issue:

When I start it and drive it in open-loop mode, it doesn't miss a beat. Runs perfectly.
When I sink the boot in or really nail it (WOT), it doesn't miss a beat. Just thumps out the power and howls up the RPM scale.

But:

When it's warmed up, in closed-loop mode and at low to mid rpms, if I just moderately open the throttle, it burps and chugs and gags and carries on until I either back right off or sink the boot in. If i really eeeeeeeeease the throttle on it doesn't complain, but I can't drive like Grandma the whole time. If I continually drive it like I stole it, it doesnt complain, but I can't do that either! Another way of describing this fault - at lower RPMs, if I drop the manifold pressure a bit (but not dramatically) with throttle, it protests.

Tests done to date:

I've replaced a new O2 sensor with another new O2 sensor - no change in fault. O2 sensor not damaged or sooty.
I started out running 27lb injectors - I swapped these for 24lb - no change in fault.
The IAT sensor is just a little off spec, low side. Cold morning (about 10C) and it was 16k ohms.
I disconnected the PCM and injected a regulated 5V signal into the TPS and MAP supply line. TPS range was 0.6V (idle) to 3.7V (WOT) and the range appeared smooth on a DVM.
MAP was on spec for the ambient (4.5V) pressure and if I really pulled all the air out of the connecting hose (that fuel residue doesn't taste very nice) I could just get it to 2.1V. MAP and TPS are original XJ donor bits.

Other considerations:

I can't find any manifold leaks, including in the brake booster. It doesn't mean there aren't any, but the perfect open loop operation and clean idle suggest to me that this is not the cause. I can't find any ignition fault and the very narrow circumstances under which it misbehaves doesn't point to coil etc even though this is an original XJ part.
I've temporarily grounded PCM line #29 so the PCM thinks the brakes are not being applied, until I can integrate the XJ's brake switch (haven't had time) This changed the way the engine behaved when pulling up at the lights - it now waits until VSS goes to zero then neatly steps the idle down to the correct RPMS - but did nothing to change the fault.

A couple of theories:

Is the 4.0 TPS for an AW4 Auto a different resistance to the one for a manual trans? With the NV3550 I no longer have the TCM in that circuit.
Going into a CJ, I've tried to properly ground everything to avoid earth voltage loops, but now I recall the PCM earth is to the LH fender while the sensor earths are on the engine block. I've bonded block to body but could there be an error voltage creeping into the O2 sensor circuit?
Fuel economy seems within bounds for a stroker still running in with less than 1000km on it: 14.5-15litres/100km (15.7-16.5MPG) in city traffic. This suggests no overall bias to rich or lean?
I have a 180 deg F thermostat in. The engine warms up properly and goes into closed loop mode within a minute or two, but it's winter here in Australia and mornings are 5-6 deg C (41-43 deg F). I have a brand new three core triple flow radiator so cooling is very effective and the engine won't be getting far over the thermostat temp of 180 deg F. Could this contribute to it?

Any other ideas? Any help greatly appreciated!

Have you tried swapping in a 190* t-stat to see if that changes anything?
Also, (and this is just a brainstorm as I'm not familiar with EFI much), but when it goes to closed loop, are there any CTO switches or similar temp-controlled switches that might be leaking vacuum? Can't hurt to throw a vacuum gauge on - even if there's no leak, the reading when the fault occurs might help diagnose it.

It's a good idea, checking the manifold pressure. I'll give that a go before dumping fresh coolant to swap out the t'stat. It's a chore for me to do that - because I have Yella Terra roller rockers (forgot to mention that!) I've fitted Hesco valve cover and thermostat housing spacers - there's two gaskets in under there and it's a fiddly PITA job to reseal it all.

The other thing I haven't done yet (but should have) is test the MAP and TPS with the PCM powering them instead of my bench supply.

Will also try looking at warm-engine O2 sensor output using an oscilloscope but I can only do that in the driveway, not under load, so can't replicate the fault conditions.

I yielded to the temptation to start throwing new parts at it today - TPS and IAT sensor on their way. (I'm assuming there's only one type of TPS..!) Lucky dip. Maybe one or the other will nail the problem....I can only hope.

You said the donor was a 94 with an aw4. Did you use that ecm? The ecm's are different for a manual trans. I'm not sure how to make one work properly with the other, but it could be that.

Hmmm - ok. Might file that one under "if all else fails". The 94 FSM doesn't mention any difference betw man and auto PCMs but of course that doesn't mean there wasn't! Now I think about it they must be different as the upshift indicator pin is missing from my PCM's connector.
I've looked after the obvious difference which is the neutral safety switch. I grounded the starter relay at the PDC, cut the wire to the PCM and left the PCM end open cct which is as per the YJ OBD1 system diagram.
The difficulty here is that 1994 AW4 auto XJs were the only OBD1 Jeeps ever sold Down Under. Got no access to a manual PCM or any idea of the part number. I really hope that isn't the problem!!

if you ask nice you could maybe buy one from someone here?? could probly work, im not sure though, maybe theres programming differences? someone more familiar can elaborate im sure

Good point. If I get to that, I promise I will ask nice! Maybe it's wishful thinking, but I'm not yet convinced that's it.

I had a look again last night at the system and wiring diagrams for a manual '94 YJ and an auto '94 XJ. The differences are in things like having cruise control and an aux fan in an XJ, but not in a YJ. If anything, the YJ PCM just looks like a dumbed-down version of the XJ - it has fewer peripherals to manage. Beyond that, the core setup is the same. My approach has been to set this up so that the PCM is "duped" into thinking it's still in the XJ and I'm pretty sure I haven't missed anything there. It's certainly not throwing any trouble codes.

My replacement TPS and IAT sensor came today so I'll try those, plus go over the earthing and test PCM output on the weekend. Will report back....!

Thanks for all the help thus far gents - really appreciate it, you're making me think harder about this! Dom

Ok - got an update on this, it might be of help to someone so I thought I'd write it up......

I checked distributor indexing - spot on as per '94 FSM and Dino's recipe.

Then, at a local Jeep dealership near where I work, I finally got a Chrysler "dealer-level" DRB plugged into the OBD1 diagnostics port to look inside and see what was going on. This DRB recognised the '94 XJ PCM's firmware and was able to see right inside OBD1 and stream live sensor readings to the display while I drove it. Wow - I've always wanted to do this..

Firstly, the PCM inputs had been set up correctly so that it operated just like it was still in an automatic XJ with ABS. I'd temporarily earthed PCM line #29 (until I get around to installing the XJ brake switch) so the DRB displayed brake status RELEASED. I'd left the park-neutral line to the PCM floating, so the "pull-high" PCM input circuit correctly came through on the DRB as D/R (drive/reverse) mode. The PCM either didn't know (or didn't care) that it was in a manual CJ with no active CCD link to the ABS or connection to a TCM. So I'm happy to report that auto/manual swap is not an issue.

Everything was sweet as a nut - except the O2 sensor readings!

Under normal closed-loop conditions the system "hunts" around the ideal Lambda value (about 0.45V I think) for the O2 sensor. It can never permanently "nail down" the perfect stoichiometric mix because other variables (load, throttle pos, temps etc) are always changing. So it tries to keep the short-term average correct by making small steps in either direction: rich-lean-rich-lean-rich-lean and checks that the O2 sensor is showing hi-lo-hi-lo-hi-lo volts. If it gets a string of hi-hi-hi-hi-hi-hi then it concludes the injector pulsewidths are off centre (towards rich) and adjusts the short term adaptive fuel trim.

As an aside, I was surprised to see that OBD1 holds both short- and long- term adaptive fuel values so OBD1 does adjust to some degree to cope with permanent rich or lean conditions, plus probably adapting to the driver's style. I didn't think it had that level of smarts!

Anyway, the problem appeared to be that the O2 sensor wasn't just taking small steps either way at low revs. It was swinging wildly to either extreme, upsetting the short term adaptive values (these were all over the place). This didn't seem to be an issue at higher RPMs - the system seemed to steady itself. But at low RPMs in closed loop, the injector pulse width was too short to be controlled by the default steps the PCM uses. The DRB consistently displayed "LOW OXYGEN" while the engine was stumbling along.

Hmmm - too rich. And all this time I thought it was too lean!

Interestingly, the long-term fuel trim was only slightly negative (leaning it out). Because I don't linger in the RPM band where the engine stumbles I don't think it has the chance to work further towards the negative end of the range.

So - to solutions. I started out with 27lb injectors, went to what 5-0 Injectors sold me at great expense as 24lb (but came in a bag labelled 25!) and the Hesco adjustable reg is checked and working in the range 34-44 psi depending on manifold pressure.

I'm going to get the stock XJ injectors cleaned, put them back in and get the fuel trims zeroed. To compensate for the requirement for more fuel in open loop mode for cold, acceleration and WOT, I'll reinstall the MAP sensor adjuster I took out for the above test, and relocate the IAT to the cold air intake.

I've also purchased a piggyback fuel-air ratio display in kit form; it's designed to work with the same voltage range for the Lambda sensor fitted to Jeeps. It has a very high input impedance so won't interfere with the PCM reading the sensor.

Should take me a couple of weeks and I'll report back...

Good job on monitoring the live data and using the info to adapt. Someday I'd like to get mine on the DRB to make sure all is up to snuff before I take a long drive to Alabama and get her tuned properly.

I’m very curious as to the end result of your experimentation. Your problem describes mine to a T, however my problem is that I am running an 89 XJ with the antiquated Renix computer. I am running a 4.6 stoker with a not to wild crane cam ( I don’t have the number in front of me). The engine itself is a typical stroker with a lot of port work. I am running the pre HO head and manifold with an HO TB with a non-HO TPS modified to fit. I am running 24# blue top ford injectors. I am getting proper info from the TPS and all other sensors are working and have been swapped with my spares just to be sure. Yesterday just because I unplugged the TPS and the engine smoothed right out. All of the bucking and messing around under light throttle was gone and my throttle response and drivability have improved. However I know that it should not be running good without the TPS and I know the computer is not going to WOT until it gets high in the RPM range. Any input would be much appriciated

G'day Justimizer,

Interesting. I think when you unplugged your TPS you might have forced your computer into open-loop mode. Again, very similar symptoms - when mine is open-loop it runs perfectly at all RPMs using the pre-determined fuel tables. It's just that closed-loop process of hunting around the O2 sensor's lambda value that I suspect causes too much fuel to be delivered at low RPM on the "rich" end of the rich-lean-rich-lean etc fuel trimming process. Well, at least that's the theory.

I have to admit I haven't progressed this yet. My injector guy told me three of the original XJ injectors I had were U/S, so I've been scratching round for replacements. It's Winter here, went skiing last week so I'm broke, waiting for some cash to come back in. I'll keep going with it and hopefully we'll both get it figured out!