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February 2007 4x4 Tech Questions - Techline

Posted in How To: Tech Qa on February 1, 2007 Comment (0)
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Techline
Four Wheeler
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Los Angeles, CA 90048-5515.

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Question: In regards to the letter titled, "Olds 455-to-K-Truck Swap" (Oct. '06), the '73-'87 generation of GMC K-trucks came equipped with the ill-fated Olds 350 diesel for a few years, which uses the same motor mount pattern as all '65-and-up "true" Olds V-8s. Just junkyard-shop for the proper motor and frame mounts.
Dana Reynolds
Chesapeake, VA

Answer: Wow, I don't know why I didn't think of this myself. You're totally right-they did use the diesel version of the 350 Olds gas motor in the '78-'81 time frame. Total brain fade on my part. Thanks for bringing it to my attention. I really appreciate it when readers help me out. The only problem with this is, it's my guess that it's going to be very difficult to locate one of these trucks as there were not a lot of them that came equipped with this diesel engine.

Question: My OEM tires (P265/70R17) on my '05 Chevy Silverado 1500 4x4 are bald already and I'd like to upgrade in size. With the bigger tires and different rim size (probably LT305/70R16) will I lose my ABS, or is that computer reprogrammable?
Michael Chachakis
via fourwheeler.com

Answer: Yes, the computer can be reprogrammed to handle the larger tire size and should be something that the dealer can do-but maybe you don't have to do so. Check with the tire dealer and see if there is any real difference in tire revolutions per mile between the two. It shouldn't be much, as the 265 is about 32 inches in diameter and the 305 is about 33 inches.

Dennis Franklin, my source on subject matters like this, tells me that you will need a 17-inch wheel to clear the brake rotor, as well as doing some fender trimming.

Question: I need to rebuild my burned-up 350 short-block and I've been looking into stroker rebuild kits. The 383 seems practical and affordable, but I've seen some claiming a 407 from a 350 short-block. Is it practical to do this to a 350? I think it would cost too much to build up a block that shouldn't be (or isn't) capable of such displacement, although cost isn't too much of a problem. I just don't want to waste money on an engine if it's bound to blow under that much stress.
Ryan Martin
via fourwheeler.com

Answer: Yep, you sure can, but it gets pretty darn expensive in order to ensure reliability. You can build a really nice 383 that will put out in the neighborhood of trail-friendly 330 to 350 hp and 400 lb-ft of torque using all factory parts, such as a modified 400 crank and rods, for a fairly reasonable amount of money. These motors really work great off-highway, and in fact, I have owned two of them, both installed in Jeeps. The only downside of using the short rods with the 400 crank is there is more side-loading of the pistons to the cylinder walls. This means the engine life won't be quite as long as if you had gone with the longer rods and custom pistons. However, it's not something to worry about.

Question: I have a '93 F-150 with a five-speed (the Mazda one, I think). I know now this tranny does not have a good reputation, but unfortunately I bought the truck before I knew this. Anyway, the problem I have is that it is very sloppy when shifting, the stick moves all around (even while in gear), and sometimes the shifter gates are hard to get right. It is extremely sloppy in this regard. Also, when I let off the gas to coast to a stop, especially at low speeds, it sometimes jerks really bad. It's like there is too much play in the gears. I don't know what the deal is-it just seems like it is out of spec or something. If I should get it looked at, I will do so, but if these problems aren't affecting the life of the tranny, then I can learn to live with it and let it go. It doesn't bother me too much any more, as I've learned to live with it already really. I don't want it to die on me if there is something I can do now, especially if it's as simple as changing a rubber bushing like other transmissions with similar problems.
Catmanduecker
via fourwheeler.com

Answer: If it is the transmission jerking, then you definitely should have the trans replaced. However, I doubt that this is the cause, but it's more likely something to do with the engine. The slop can be caused by several things, from a worn selector to shift forks to gears and bearings, and not just a worn-out rubber bushing. It's a pretty good sign that the trans is on its last legs. Consider a rebuild in the near future.

Question: I have a relatively stock '78 Chevy K-10 pickup. It has a 4-inch suspension lift and a 2-inch body lift so I can fit my 35-inch mudders under it without cutting anything. It has an NP205 transfer case, and the problem is, the last couple times I have put my truck into an extreme situation (steep hillclimb up mud and grass, loaded with 1,400 pounds of dirt, or just trying to get over a few boulders), the front wheel or wheels begin to spin and the back end just sits there, as if it is somehow in front-wheel drive. Two-wheel drive works fine, and it even drives in four-wheel drive in an easy situation. But under stress, all of the power goes directly to the front wheels. Where do I begin diagnosing my problems?
Hunter Fox
via fourwheeler.com

Answer: There could be a couple of reasons why your transfer case won't send power to the rear wheels.

First off, are you sure it's a 205 and not a 203? The trucks built in 1978 had an automatic transmission with the 203 and no front locking hubs. The 205s came with a standard transmission. Or, it could be, now that your truck is approaching 30 years old, someone before you bought the truck and could have done a transfer-case swap.

The 203 is a full-time unit with a 2-Hi, 4-Hi, 4-Hi Lock, and 4-Lo Lock position indicated on the shifter. It's possible that if someone had swapped out a 203 in place of the original 205, then the shifter would not show this pattern. When the 203 is not in a "Lock" position, power will be routed to the wheel that has the least traction. In the instance you described, where you would be going up a muddy hill with a load in the back, the front would have the least amount of traction and you would notice that the left front was spinning.

The 203 is a physically larger case than the 205, but if you don't have the two cases to compare against each other you may not be able to tell the difference. The 205s do have an I.D. tag on the front of the case, but it's difficult to read when it is in the truck and covered with grime. The easiest way I have found to tell the difference is the small idler-shaft plate that is held on with three bolts on the back of the case between the rear output housing and the large aluminum front output bearing retainer. Only the 205 has this cover plate.

Now there is another possibility. According to NP205 guru Stephen Watson of Off Road Design, (www.offroaddesign.com), if there is a lot of wear in the shift rails as well as the internals of the transfer case, it just is possible that when shifting into low-range four-wheel drive that the shifter is not allowing the rear drive gear to properly engage. It's rare, but he has seen it happen. Again, this may be possible because of the age of the truck and especially so if there are a lot of miles without a rebuild.

Question: I own an '84 GMC truck that I built up with a Cummins diesel and a divorced NP205. The brakes are vacuum-assisted, and I used the stock 1/2-ton Chevy booster and am using the vacuum pump off the diesel engine. I don't have a strong pedal-instead, it feels spongy. I have brand-new calipers in the front and new wheel cylinders in the back, and the rubber lines are new. I have tried three different master cylinders.

How much vacuum does it need to work properly? Is the diesel engine pump providing the necessary amount of vacuum?
Franco Collart
via fourwheeler.com

Answer: To start with, the first thing I would do is get a vacuum gauge and check the pull of the vacuum pump. If it's in good condition, it should put out some where between, say, 17 to 19 inches of vacuum. In reality, your power-brake booster should work just fine with 14 inches and most likely down to 12, and in some instances 9 or so.

A bad master cylinder or even a bad power brake unit will generally not give you a "spongy" feel to the brake pedal. This usually comes from air trapped in the line. I will assume that you did the proper bench bleeding as per the instructions that came with the master cylinder. It's not uncommon to have air trapped within the system some place, especially in the calipers.

I have tried all sorts of different ways to bleed brakes, but my favorite way is to attach a clear hose to the bleeder screw and run it into a container. Have someone push down on the brake pedal slowly while you open the bleeder screw just enough to let the fluid flow. Loosen the screw too much, and you take the chance of air leaking around the threads and giving you a false impression of never being able to get all the air out of the caliper or wheel cylinder. With the pedal on the floor, close the bleeder screw. Have the helper raise the pedal and count to 10 before pushing down again. This gives the system a chance to refill. Do this until you are positive there is no air in the system. This is the nice thing about the clear line as you can see any air bubbles coming out. You may have to refill the master cylinder several times. You never want to empty the master cylinder of fluid as then you're in for a long time of bleeding. Oh, and don't leave the cap off the master cylinder or the brake fluid can as it will pick up moisture from the air.

You may want to tap the calipers with a soft hammer as you're bleeding them to release any trapped air. Sometimes it is even necessary to take the caliper free from its mount and turn it so that the bleeder screw is at the highest point during the bleeding process. Sometimes it's necessary to find the combination valve and hold the metering valve pin in an upward position while bleeding, but not always. They make a special tool to do this with, but a pair of locking needle-nose pliers will work also.

Question: I have a '91 Nissan Hardbody King Cab 4x4 with the 2.4L four-cylinder, five-speed, basically stock drivetrain, K&N air filter, and torsion-bar crank (1- to 2-inch lift) in the front, 31x10.50x15 BFG A/Ts and straight-pipe exhaust with 20-inch glasspacks and 16-inch echo tip that it had when I bought it. The engine was rebuilt about 25,000 miles ago.

Mostly it is a daily driver and weekend warrior going on just mild trails and mud running. The biggest problem I am having is a lack of power. While I do not expect a lot out of a four-cylinder, this just stinks. I've heard that backpressure is everything for a four-banger and I need to put a muffler back in and to add new gears. I would like a deeper, throatier sound anyhow, rather than the bumblebee it is currently. I can't run much over 70 mph and the acceleration sucks, especially on the top end, but it lacks power all around. Hills and wind are horrible and interstate (75 mph) travel is nearly impossible. It struggles sometimes climbing steep slopes in low gear.

I had almost given up and figured it was hopeless, but I really like the truck. I see Toyotas run bigger tires and seem to do fine. Nissans, especially Hardbodys, are not very popular here. My goal is to improve its trail capabilities with a lift and 33- or 35-inch M/T tires, but I'm afraid of making things worse.

The gears are about $1,000 and I am kinda avoiding that unless that is what it has to have. There are also a few upgrades like cam/rockers, ECU upgrade, performance header, performance intake, but these are also approximately $1,000, and turn only about 20 hp more. I'd like to keep my four-cylinder to avoid doing a swap and the downtime of a daily driver. Can you help?
Nick Forrest
Miami, OK

Answer: I doubt that the lack of "backpressure" in the exhaust system just because you removed the stock muffler is causing a loss of torque. I have no idea where this myth came from, but in your case it's totally false. Yes, in some instances a better-flowing exhaust can cause a lean fuel condition, and maybe a loss of power, but your fuel injection should handle the minor change in air/fuel ratio without a problem.

Yes, you could put the money, like you said, into the motor and make about 20 more horsepower. In reality, that is a pretty big percentage boost, but it is not going to get you the performance that you want. In fact, it may kill the performance. Why? Because all the horsepower gain will be in the upper rpm range, not down low in the powerband where you need the extra power to get and keep those large tires moving.

I would really recommend that you invest some money in some lower gears. Automotive Customizers (954/971-3770, 4x4parts.com) has the ring-and-pinion gears you will need. They are available in 4.375:1, 4.625:1, and 4.875:1 ratios. The price is in the $450 range for each gearset, so by the time you pay someone to set them up, you're going to be over the $1,000 mark.

In all reality, you should also go for a locking differential-at least in the rear-while you're doing the gear swap. You will be totally amazed at what it will do for your Nissan's off-highway ability.

What gear ratio to go with? I am leaning towards the 4.625:1 gearing with the 31-inch tires, which will buzz your engine at about 2,600 rpm at 65 mph in top gear. The 4.875:1 gears will bring the rpm up to about 2,900 or so, but it would be a much better choice if you plan on going to a larger-diameter tire in the future. Just keep in mind that the lower gearing means less gear strength because the tooth contact area of the pinion gear goes down with the smaller tooth count. With the four-cylinder engine I doubt that this will be a problem.

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