The AMC/Jeep 4.0 liter inline-six: Synonymous with reliability, these engines are well known to easily last to 300,000 miles or more. They endure torture in off-road applications, overheating, water ingestion, low oil pressure from extreme angles and shrug it off like it never happened. They make decent power (ranging from 173hp-190 hp) and gobs of torque. What better possible endurance racing engine? That’s what I thought when I selected the Jeep Cherokee as the platform for the Petty Cash Racing 24 Hours of LeMons racer.
Before I explain how terribly misguided this assumption was, a little history is in order. In 1964, AMC replaced their previous Nash derived OHV I-6 with the basic foundation that survived until 2006 in continuous production. So this lump of iron has been around the block more than a few times, and like I mention, earned a sterling reputation for reliability, especially in its final iteration, the Chrysler influenced 242 ci/4.0 liter version. At first the team had very good luck with the 4.0. Our first race was Thunderhill in 2009, and the Jeep surprised a LOT of people that first outing. We actually had to get a special exemption from organizer and self-proclaimed “Chief Perpetrator”, Jay Lamm, just to enter the event. People were convinced we were going to roll over on the first corner.
Instead, we finished 63rd out of 156 entries, a convincing result considering none of the team had ever been on a race track in competition before. The motor did great, didn’t use oil and even sounded mean with our Cherry Bomb muffler and side exit exhaust. We thought we were pretty clever.
Things had gone so swimmingly at our first race, we decided to not change a thing and raced at what was then called Infineon Raceway, aka Sears Point. We finished mid-pack due to some unscheduled trips to the penalty box for contact with other cars. Up until this point, all we’d done to the Jeep was change the oil and brake pads. So far so good. In fact, things were going so well that we decided to leave it mostly alone for our next race at Buttonwillow Raceway near Bakersfield in December of 2010. We were baffled when we developed a pretty serious case of blow-by, and enough crank case over-pressure to blow out the dipstick out of its tube (enter zip-ties). Despite the slightly wounded engine, we took home a trophy, first place in Class C, this time 32nd/173 entries.
Our next race was at Sears Point again, and we were looking forward to redeeming ourselves. That lasted 3 hours and then a piston exited block-left. We were baffled. 4.0s are supposed to last forever! No matter that we bought ours with 201k miles and had proceeded to put about 3500 racing miles on it. We scrambled all night and swapped in a junk yard motor that we found that night and finished the race, but we were now a little suspicious of our old friend the 242 straight six.
Cut to 2011. We knew the Jeep was capable of some pretty decent finishes, and I’d been bitten by the racing bug. 2011 was going to be our year, an all out assault on some serious results. I broke out my Sponsorship Knee Pads and went to work: We got a set of massive brakes from Savvy Off-Road loaded with Black Magic pads. We added 2 degrees of negative camber on each front ball joint. Perhaps the biggest improvement was the selection of new 17″ Raceline Wheels wrapped in the new Falken Azenis RT-615Ks in lieu of the 340 TW Mud & Snow rated garbage we had . Disc brakes, limited slip differential and shorter gears in back courtesy of a Ford Explorer rounded out the package. It was a different car.
We went back to Sears Point with high hopes; now we would have the handling to match the reliability and relative speed we’d been enjoying for the last year. And blew a motor 3 hours in, just like last time. It’s almost like there’s something about this track…like a specific corner that robs the motor of oil pressure. Like maybe the long, downhill, off-camber sweeper called the Carousel. Hmmm.
Luckily, by this time we were wise to the 4.0s proclivity to eat it’s own bottom end (insert joke here) and brought a spare.
We took the checkered flag for that race, but it was a not one of our more fun events. Lots of time spent in the pits, not much time actually on track. I swore that we would not return until we had solved our engine issues. Post race video shows our oil pressure dropping dramatically through The Carousel. You can hear the motor getting worse and worse in this video:
Clue #1. Maybe our increased grip and higher lateral Gs due to our upgrades have made oil slosh in the pan the problem. But that can’t be! I’ve run 4-liters their sides’ for minutes at a time in off-roading with nary a problem.
But then it struck me…I was idling at those angles, not running an engine designed with a 5000 RPM redline at 4600. So what to do? I put out a plea on our Facebook page and was flooded with suggestions: “Run 2-stroke oil!” “Add saw dust!” “Put no oil in at all!” “Make your roll cage part of the oiling system!” “90 Weight gear oil!” Out of all these terrible ideas, a couple floated their way to the surface. First, add baffles to the oil pan to keep oil from sloshing around under high load corners. Some CAD (Cardboard Aided Design) later, and we had a partial solution:
Another idea that some old dirt track guys suggested was an Accusump. Basically, it’s a reserve tank of oil that is forced into the engine when it senses low oil pressure. These are surprisingly cheap, and we found a used one in the 2-qt flavor on Craigslist for $85: Insurance plan #2.
We also added a mechanical oil pressure gauge and an oil temp gauge so we could keep better track of what was going on under the hood, hopefully before parts tear an inspection port in the side of the block.
While the motor was apart, we tossed in a new set of rod bearings to further bolster our chances of making it through our next race, appropriately enough at Sears Point. Thanks to Eastside Auto Care for helping us out on the parts.
Perhaps the most contentious question ever asked is “What kind of oil should we be using?” We got another onslaught of suggestions: “The thinner the better!” The Heavier the better!” “Distilled baby seal fat is the only way to go!” “Make sure you over fill it by 2 quarts!” Whatever you do, don’t over fill it! You’ll blow up!” We got 81 answers on our Facebook page in an hour. It was pandemonium. One person stood out of the crowd, however, and offered up some very constructive and well thought-out ideas. Turns out the guy, Dirk Werning, is not only a Jeep Cherokee geek, he also runs 4x4Oil.com, an AmsOil dealer. It turns out the Mobil 1 Synthetic we’d been running really wasn’t the right solution for us.
Apparently motors with flat tappet valvetrains need something called ZDDP (Zinc dialkyldithiophosphate), basically a zinc additive to keep the engine from chewing itself up. Dirk advised us that with the high miles our crappy motors had, something a little heavier would be the way to go and shipped us a case (along with a couple AmsOil filters) of AmsOil 20/50 Synthetic Z-Rod. You can read more about ZDDP on their site. Interesting stuff.
So, with seemingly all our bases covered, we loaded up for our next race at Sears Point.
Success! We finished a race at Sears Point without having to do a motor swap! I can already hear you saying it: “One race? That’s hardly conclusive.” Agreed; we decided to put the AmsOil to the ultimate test. A normal LeMons race weekend is about 15 hours of racing broken up over 2 days for a total of about 800 miles. Once a year, however, they do a true 24 hour enduro, straight through the night. You may remember that we won last year’s event in Reno. This year’s full 24 was going to be at Buttonwillow, near Bakersfield, California. Temps were predicted to be above 100 degrees. Perfect torture test.
Even with engine coolant temps soaring into the 250+ range and over heating, oil temps never got higher than 280 and we never once saw low oil pressure. That’s on the same oil and filter from the previous race, for a total of 2300 race miles. Okay, we’re officially drinking the cool-aid.