Straight talk from the TL tech team

Vein Vortec II

We have had identical problems to those described by Mr. Ebeling in an earlier column (`Vexing Vortec'), such as rough running, jerking and surging. We have a 1997 GMC 2500 with a Vortec 7400 engine, automatic transmission and 3.73:1 axle ratio, and we tow a fifth-wheel trailer with almost the identical weight. Our truck has even been driven by a repair technician while towing the trailer. The problem is acknowledged by the local dealership, but only hinted at by GMC. The computer shows nothing, also. The dealer told us that the drivability problem (jerking) wouldn't hurt the drivetrain.

We appreciate the information regarding an aftermarket cure. However, we feel that GM should bear the expense, not us.

BOB PEHL MERCED, CALIFORNIA

Since I wrote that earlier column, I have found some new information on this problem. At cruise speeds on level roads, the engine is typically running with 30- to 50-percent throttle. The computer allows the engine to run a lean fuel mixture under these conditions. The engine may vibrate (GM calls it a "chuggle" in technospeak) due to the lean mixture, particularly with a trailer in tow. At about 70-percent throttle opening, the computer begins to enrich the fuel mixture and the "chuggle" should go away.

The torque converter is also locked up in that driving mode, which transmits more of the vibration to the vehicle. To determine what effect the torque converter has, you can briefly unlock the converter by tapping the brake pedal with your left foot while holding the gas pedal steady with the right foot.

Your truck's computer has onboard diagnostics version two (OBDII) with flash programming. Have your dealer reprogram your computer with the latest GM CDROM; it must be at least version 16 or newer These CDs have better calibrations for 1996 and newer models.

There's also a possible problem underneath that needs to be checked. The longerwheelbase trucks, such as extended cab and Crew Cabs, have a lot of frame flex. As a resuit of this flexing, trucks with dual catalytic converters can develop air leaks between the cats that cause false readings in the rear oxygen sensors. The sensors then cause drivability problems, such as surging, that don't set a diagnostic code or cause the "Service Engine" lamp to go on. If a possible leak is found, have it corrected at once.

To Tarn or Not to Tarn?

Should a travel trailer be covered with a tarp or custom-made cover for the winter? I've been told that putting a tarp over the roof of a trailer while it's in outside storage for the winter will trap moisture under the tarp with each freeze-and-thaw cycle, and this process will degrade the roof. I've also been told that covering the upper part of the trailer will greatly lengthen the life of the roof, and protect the air-conditioning unit and the various vents and antennas.

Is there a benefit to having a heat source (light bulb) inside the unit during the storage period? Some people claim that having a light bulb burning inside the trailer's enclosed space will limit the growth of mold and mildew. If the unit has been properly cleaned prior to being parked for the winter, is there a benefit to having such a light inside the unit?

GEOFF LIVINGSTON YORK SPRINGS, PENNSYLVANIA

* There is definite benefit to be gained from using a cover, as long as it's designed for RV use. Most inexpensive, all-purpose tarps can create moisture problems as described in your letter The light bulbs work, but their use has hazards involved, such as fire, so they aren't recommended. As you suggested, the cover helps protect the roof, air conditioner, vents and the like from water or snow or ice damage.

If the cover extends down the side walls and covers the windows, leave a few of the windows and a couple of roof vents cracked open to allow some air circulation. Since they are protected by the cover, no rain or other moisture will get into the vehicle, but air will be able to circulate and help deter mildew and musty smells.

Cold-Start Knocking

I have a 1994 GMC van with a 5.7-liter engine that has 48,000 miles on it. Sometimes I have a little knock for about five seconds when I first start it and also on restart after sitting about 30 minutes or more. My dealer replaced the main bearings and connecting-rod bearings (with standard-size items) for my piece of mind at 37,000 miles, but it still does it. The dealer also suggested that I use an AC/Delco no. 1218 oil filter and try 30-weight motor oil. I know of several people with the same problem. Do you have any suggestions?

MICHAEL MAC GUYMON, OKLAHOMA

* Engine knock on cold startups has been a fairly common complaint on GM V-8s for some time now. What happens is the oil pump seems to lose its prime after the engine sits for a while, usually overnight. GM's policy in the field seems to be that if the knock occurs for less than 10 seconds, there is no problem.

Your dealer's advice about using an AC] 218 oil filter was correct. This filter has a check valve that helps prevent oil drainback. Filters such as the PF35 and PF51 don't have anti-drain-back valves. Some people report an improvement when using thicker oil, such as straight 30- or 40weight, but be sure to use multi-weight in cold weather, because thick oil won't circulate as quickly on startups. Try your dealer's suggestions; if they solve the problem, great.

In many cases, though, the culprit seems to be the baseplate in the oil pump that the gears ride against. The GM pumps have a fairly rough machined surface that allows the oil to leak down overnight. Quality aftermarket pumps, such as the one made by Melling, seem to solve the loss-of-prime problem and get rid of the knocking in some cases where bearing changes and oil filters with check valves didn't solve it.

Tire Size & Blowouts

I've had a problem with the tires on my 1998 32-foot Terry Resort fifthwheel trailer. I purchased the trailer in March 1998 and have had two blowouts so far, running fully inflated and with no abnormal wear. I have a friend who has the same tires on his new 32foot Jayco fifth-wheel, and he's had three blowouts in two years.

At the last RV show I attended, I spoke to several dealers and was told that any fifth-wheel that's longer than 30 feet is now ordered with 16-inch wheels and tires. Is there something that I wasn't told when I bought my trailer?

JERRY BROUSSARD VIA THE INTERNET

* You didn't really miss anything,Jerry, except that perhaps your trailer tires are overloaded. That's usually the case when a tire is run at its proper inflation pressure, it displays no excess wear patterns and yet, blows out prematurely.

Here's how to find out if your trailer tires are overloaded. First, take your trailer (fully loaded and ready-to-go) to a public scale and weigh just the axles with the trailer level on the scale. Next, check your weight sticker for the trailer's gross axle weight rating (gawr) and compare the two figures. If the actual weight is higher than the combined gawr of both axles, the tires are probably overloaded.

If you find that the tires are overloaded, the first solution is to reduce the amount of of weight carried in the trailer If that's not possible, in regard to how much you want to take along versus the trailer's tire ratings, you may need to replace the complete axle assemblies, including axle, hubs, tires, wheels and springs, with those of a higher rating. That may seem like a drastic measure, but it's the best way to make sure all of your suspension parts are up to snuff and able to carry the load.

Today's higher road speeds can aggravate an overloaded-tire blowout situation. Speed causes heat buildup in a tire and excess heat plus excess weight (which helps cause the heat) leads to tire casing failure.

Many larger trailers today are fitted with 16-inch wheels and tires because they're typically rated to carry more weight than their 15-inch equivalents. Thus, along with a heavier-rated axle and suspension, the trailer can have a higher gawr to suit the trailer's larger size. It's a side benefit, but many 16-inch trailer wheels have hub bolt patterns and shapes that also fit the truck used to tow the trailer, in which case the owner only needs to carry one spare tire at a time.

Assuming your tires were not overloaded or underinflated, it's possible that, by design, they have a pattern of failure even when used properly. Also, given the same loading and inflation parameters, a specific number of tires like yours may have been defective.

More Mopar Power!

Can you help me with the relationships between tow vehicle horsepower, torque, weight and road gradient? I have a Dodge Ram 1500 two-wheel-drive (2WD) pickup with a 5.9-liter V-8, and I tow a 5,200-pound 30-foot trailer. I probably have 1,000 to 1,500 pounds of stuff in the rig when towing. It appears to me that another 30 to 50 hp would make a difference up hills, at stop signs, etc. However, an aftermarket exhaust, chip, turbo, etc., are costly and take a fair amount of time to install.

Everything is basically acceptable, except on hills. In your opinion, is it worth it to modify this truck, leave it alone or buy a new vehicle with a big V-8, V-10 or diesel? We can't just go down to the local dealer, hook up our trailer and say, "OK, I'll be checking this truck out for the weekend."

STEVEN MORAN MIDDLESEX, NEW JERSEY

* Besides advertised horsepower, torque and weight, you have to consider factors like axle ratios, tire diameters, internal transmission ratios, torque-converter lockup strategies, rolling friction, surface area and coefficient of drag, plus true weight and horsepower and torque and power curves as measured at the drive wheels. The relationship between these items is so complex that stacks of books have been written about the subject, and it's beyond the scope of this column. When all is said and done, most people buy a vehicle because they like the salesperson, or the way the vehicle looks or rides, or they think they got a super deal, etc.

Dodge's 5.9-liter Magnum V-8 makes good power for an engine of its size. You can't simply install a "chip" into your truck's computer (the whole computer has to be replaced), and I don't know of any company that markets a complete emissions-legal turbo kit for it either There are some good aftermarket exhaust systems, though.

I don't think that 30-footer is as light as you believe it is. Considering the true trailer weight plus the extra weight you carry, you're probably exceeding the truck manufacturer's gross combination weight rating (gcwr). This is a common situation that leads to poor performance and is part of the reason why we publish the annual "Tow Rating Guide" and related articles like "Weight Watching."

Take the time to read our tow vehicle tests and product evaluations of aftermarket performance enhancements; you'll get a feel for how they work, and be sure to read the test specs. If your rig's overloaded, shop carefully for another one.

Copyright T L Enterprises, Inc. May 1999
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