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  • TDC - Technical Questions & Answers - August 2004

TDC - Technical Questions & Answers - August 2004

Questions and Answers

Bruce Caldwell
Aug 1, 2004
Photo 1/2   |   tdc compression Lesson
Compression Lesson
I'm looking to buy my first truck, but I don't have a lot of money and I don't want to buy something that needs expensive repairs. Trucks that are within my budget ($5,000 or less) will most likely have at least 100,000 miles on them. What I'm concerned about is avoiding major repairs, such as a worn-out engine. I've seen friends learn the hard way; you can't always believe everything a seller tells you. Also, most buyers seem to just stare at an engine without really testing anything. Would it be a good idea to do a compression test on an engine? I'm most interested in finding a GM or Ford 1/2-ton shorty with a V-8 engine. How many miles can you generally expect before a small-block V-8 needs a rebuild? I would appreciate any advice you could give me.
Matthew Gianni, via e-mail
A: You're wise to check out a high-mileage truck as thoroughly as possible. Regardless of how carefully you inspect a truck, there will probably be some unexpected repairs. If you could set aside 10-20 percent of your budget for repairs, that would lessen the shock. The only thing that most buyers determine by looking under the hood is that, yes, an engine is present. They may look at and smell the oil and transmission dipsticks to check for obvious signs, such as water in the oil or burned transmission fluid.
Articles on buying used vehicles often recommend having the potential purchase checked out by an independent mechanic. That sounds great, but few buyers ever do it and most sellers are reluctant to let you. Why should they go to all that hassle? Besides the risk of getting a bad report, sellers don't want their truck at a shop when they could be showing it to less picky buyers. Running a compression test at the seller's residence shouldn't be a problem if you establish that you're seriously interested. There can be some knuckle scraping involved with compression tests, so it doesn't make sense to test unless you're strongly considering the truck. There should be instructions and test information with the compression gauge, but we'll cover a few basics. You should buy a quality compression gauge, the type that screws into the spark plug threads.
You should expect some signs of wear on a 100,000-plus-mile engine. What you don't want to find are inconsistent compression readings. It's OK for all cylinders to be a few pounds below spec, but you don't want big differences between cylinders. A typical small-block Chevy engine will have 150 psi as its theoretical optimum. If your prospective purchase has consistent 130-140 psi readings, that's pretty good for a well-used engine. Before starting the compression test, check that the battery is healthy. A strong battery is necessary to turn the engine over quickly and consistently for each cylinder's test. The test won't be accurate if the engine pumps slower for some cylinders than others. A remote starter switch is a good idea, so you don't have to keep shouting instructions to someone in the cab. Generally, you need to turn the engine over about six times or until the gauge stops climbing. The quicker pressure builds, the healthier the engine is.
A low pressure reading on the first revolution followed by a gradual increase is indicative of worn piston rings. If the initial low reading doesn't improve, the problem could be leaky valves or a blown head gasket. A worst-case scenario would be a cracked cylinder head. If the readings seem a little low, yet the engine seems to run fine, you can retest it. Add a couple squirts of engine oil to each cylinder before repeating the test. If there is a noticeable improvement in the readings, worn rings are indicated. If readings remain low after adding the oil, we'd suspect valve problems. Possible valve problems can include burnt valves or valve seats, warped or bent valves, and cracked valves. If two adjacent cylinders are equally low, that's an indication of a blown head gasket. You can also have the problem of readings that are higher than specified. Overly high readings can mean the engine has excessive carbon deposits
Regarding how long a typical small-block can go before it needs a rebuild, it depends on how the truck was used and maintained. A truck that has regular maintenance records or evidence of frequent oil changes (look for door or windshield stickers from a quick lube facility) is something that usually indicates engine longevity. Unless an engine was severely abused, it should make 100,000 miles without any problems. The 150,000 mark is easily attained by most small-blocks. In the 175,000 to 200,000 mile range, we start to get a little suspicious. Topping 200,000 miles isn't a problem for a well-maintained engine, but the odds of some repairs definitely increase. On really high-mileage trucks it's best to assume something will break sooner rather than later and price the truck with room for future repairs.
Vanishing Vacuum
The shift quality of my '92 Ford Explorer is rapidly declining. The Explorer is equipped with the 4.0L V-6 and the four-speed automatic transmission. The mileage is about 140,000. I'm familiar with the vehicle history and no major work has ever been done on the transmission. What's happening is that the shifts are getting progressively harder. Downshifts are worse than upshifts. This problem didn't just happen, but over the period of a year or so it's become quite noticeable. Does this mean some major transmission repairs are needed or will soon be needed?
Barry Schultz, via e-mail
A: The most likely source of your hard shifting is a defective or failing transmission vacuum modulator valve. If the diaphragm inside the modulator valve starts to leak or ruptures, that will cause hard shifting. Take your truck to a transmission shop and explain the symptoms. The shop can easily check the modulator valve and replace it if necessary.
Don't Knock It
I bought a rebuilt 350 small-block of uncertain vintage to replace the beat-to-death original 305 V-8 in my '84 GMC 1/2-ton pickup. I'm confident that the 350 is fine because I've got the machine-shop receipts. My problem is that the truck has a knock sensor that screwed into the side of the block. The new 350 doesn't have a provision for the sensor. I'd like to know if I should reinstall the knock sensor and if so, where should I put it?
Raffiq Rajabali, Indianapolis
A: You should retain the knock sensor function, but you will need one that is designed for an '84 GM truck with a 350 engine. Sensors for a 305 aren't the same as ones for a 350. Get a new one from your local GM parts department. The sensor is screwed into the block's water jacket drain; this is a 1/4-inch pipe-tapped hole along the oil pan rail. You can carefully drill into the boss and tap the hole with a 1/4-inch tapered pipe tap. If you don't feel confident about drilling into the block, take the engine to a machine shop before you install it in your truck.
Highway Hiccup
I have a very annoying and potentially dangerous problem with my '82 Ford F-150. The engine occasionally dies when I'm cruising along on the freeway. The truck has a carbureted 300ci straight-six engine and a manual transmission. Thankfully it restarts easily, but it's unnerving at best. I've replaced the carburetor and most of the ignition system, including the spark plugs, plug wires, distributor cap, condenser, and coil. Can you suggest something I might have missed or what else could be causing this problem?
John Dobner, Billings, MT
A: We think you should replace the ignition module with a new Ford original-equipment model. You've replaced lots of parts related to the ignition module, but it's the module that controls current flow. It causes the coil to send the proper spark through the plug wires to the spark plugs.
When the ignition key is in the "run" position, current flows from the battery to the coil and then to the ignition module and then back to the battery. This flow of current creates a magnetic field in the coil. At a predetermined intensity, the module cuts off the current, which collapses the field in the coil. That creates voltage, which goes to the distributor where it's routed to the spark plugs. Then the module permits the current to return to the coil, where a magnetic field is again developed and the cycle repeats itself.
You need the exact module for your truck, which is why it's a good idea to use a Ford part instead of a more generic aftermarket unit. Your module should be located on the driver side of the engine compartment by the inner fender. When disconnecting the wires, pull them rather than pry them.
Photo 2/2   |   tdc view Window
View Window
I've wanted a big-window '67-'72 Chevy/GMC shortbed Fleetside pickup for as long as I can remember. Rust-free affordable ones are tough to find in my part of the country. I've found a couple decent longbed ones, but I passed on them because I thought it would be too much trouble to convert them to shortbeds. Now I'm beginning to have second thoughts. I did find and buy a pretty straight '68 C-10 Fleetside, but it has the small rear window and I really prefer the looks of the big window. The cab has rust in the door rocker panels, but that's the only serious rust. My question to you is how much trouble would it be to convert my cab to a big window? Or, would it be less trouble to buy a big-window longbed truck and shorten the frame and put my shortbed on the shortened frame? Or, would it be worth the hassle to swap cabs? All this swapping and shortening assumes that I can find another reasonable longbed deal such as the ones I passed on earlier. I'd rather just convert my existing cab than buy another truck and go to a lot of work. I'm hoping you might be able to give me some insight into this problem. Thank you
Josh Bloomfield, Syracuse, NY
A: The easiest solution is to learn to love the small rear window. That's' what we're doing with our '67 C-10. The possibilities you listed are pretty labor- and money-intensive. A very low-cost alternative is to use paint to give the illusion of a big rear window. Tint your existing window with as dark of a tint as you can get. Then have the area on each side of the window painted black to mimic the shape of a factory big window. Unless your truck is black, the effect will look like a dark-tinted big window from a distance. To take it a step further you could have an airbrush artist paint fake chrome trim around the whole "window." You can also buy reproduction stainless trim for a really big window. You would need to install mounting clips in the correct places to secure the trim to the cab.
Of the other possibilities you listed, shortening a longbed big-window truck is easier than installing a new window. You can buy all the reproduction pieces needed to assemble a brand-new shortbed, so you wouldn't have to waste your existing shortbed truck. Shortening the chassis isn't an average home-garage job, but a shop that deals in custom-bodied commercial trucks could easily do it. Swapping cabs and reassembling both trucks is a huge undertaking. Another easy no-cutting-involved solution is to find a nice big-window longbed and build it. We've seen some very nice longbeds done in the retro-rod style with stock two-tone Cheyenne paint schemes, all the factory trim, lowered suspension, and contemporary wheels and tires (or big rally wheels).
Seeing Red
I'm building a trick '02 Escalade. I want it to be as radical as possible without compromising any of its daily transportation functionality. Toward that end, I've already 'bagged it and equipped it with some 22-inch Milos-6 wheels and BFG rubber. Rather than do any radical body mods that may or may not be fashionable next year, I've decided to paint it the wildest possible red. My question to you is, how do I find the brightest shade of red and once I find it, how do I know that it will look that way once it's on my ride? I'm willing to spend what it takes to get that perfect red, but I'm not an open checkbook. It seems like everyone I talk to has a different opinion.
Ryan Norton, Jacksonville, Florida
A: Your question is a variation on many similar ones we receive. It's also a question that we overhear at shows. As avid red fans, we're always asking truck owners what color of red they used, what kind of paint it is, and who painted it. The only way you're going to get the absolute exact same color is to have it prepped exactly the same way and painted at the same time, by the same painter, using paint mixed in one big container. Then you need to have it cleared, color-sanded, and buffed exactly the same way. In other words, matching paint is tough
You can get close, though, by using the same procedures and products as the paintjob you're trying to copy. Remember that reds you see in magazines won't necessarily look that way in person. Photography and printing variables can affect the way a color looks in a magazine versus how it looks in person. Even colors you see at shows can vary under different lighting conditions. Indoor shows are notorious for colors that look different in natural light. Remember that at outdoor shows your polarized sunglasses make a big difference in how intense colors appear. It would be great if you could show your truck inside a polarized case. Light is all-important when matching colors
In determining which red you want, you need to decide what hue of red you prefer. There are reds that lean toward orange and reds with a blue cast. Basecoat color can have an effect on the final color. Some custom painters claim to have their own secret formulas, but that can be as much myth as fact. The top paint companies have the resources to formulate the most vivid, long-lasting colors, so you're as well off using an off-the-shelf color as some secret formula. The benefit of using factory colors is the added ease of repairs if you ever need them.
Custom paintjobs on a vehicle such as yours aren't cheap. You're looking at a total color change, which means a great deal of time must be spent on the doorjambs, tailgate areas, and under the hood. Since this is a substantial investment, it makes sense to spend the extra money required for test panels. Paint chips are a poor way to judge a color. You can tell if you're in the neighborhood, but a test panel done with the appropriate underlying materials and number of topcoats is the best way to judge a color. The right red is tough to beat. Good luck in your quest.
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