Q: I think one or both of the exhaust manifolds on my '96 Chevy Silverado are damaged. The truck has the 5.7L V-8 and the automatic transmission. It has more than 140,000 miles on it. I've had bad exhaust manifolds and headers before, so I'm pretty sure this is the problem. It definitely sounds like a bad manifold. I'm quite certain that the right manifold is cracked, but I'm not sure about the left one.
I tow a 24-foot travel trailer in the summer, so the truck gets some pretty hard use, especially when crossing mountain passes. Can hard use contribute to cracked exhaust manifolds? Would headers be a better idea? If I find that the exhaust manifold is cracked and I have it welded, will it just crack again in the same place? I'd appreciate your advice.
Barry Noonan, Butte, Montana
A: Cracked right-side exhaust manifolds have been a problem for '94-'96 GM pickups with the 5.7L V-8. Extreme-duty use such as towing a heavy trailer long distances in hot weather will exacerbate the problem. The manifolds get very hot and then cool off. The technical term for this condition is thermal cycling stress. These extremes of temperature can lead to stress lines, which eventually become full-fledged cracks. Sustained high engine speeds can also contribute to exhaust manifold cracking.
General Motors has a couple revised exhaust manifolds to fix the problem. Engines that were built in 1994 and early 1995 require right-hand exhaust manifold PN 12524289. A way to identify the earlier exhaust manifolds is that they have separate AIR tubes screwed into each individual exhaust runner. Later engines such as yours use exhaust manifold PN 12556731. These revised manifolds use a single AIR port that's located at the front of the manifold.
You could have the crack (or cracks) welded. The quality and longevity of the repair have a lot to do with the skill of the welder. We'd skip the risk and just get a new, revised manifold. Headers would be a fine alternative and would give you extra power for trailer towing.
Q: I have a '90 Ford XL longbed pickup with the 5.8L 351ci engine and four-speed automatic. The mileage is fast approaching 200,000, but no major work has ever been done to it. It runs fine, but does tend to use oil. This truck has become sort of the family chore truck. Everyone borrows it whenever they need to haul something. I'm fine with that, as long as they bring it back full of gas and watch the oil consumption.
My reason for writing you involves the oil consumption. I tell everyone who borrows the truck to keep close tabs on the oil level. A young relative used the truck to move 600 miles away. He had the truck for almost a month. When it was returned, I checked the oil level and it was over-full. I changed the oil and my best guess is that it was 1 to 1-1/2 quarts over-full. I don't know how long he ran the truck this way.
My question is: Can over-filling the oil harm an engine? The truck runs like it did before, so there isn't any obvious damage. I've only admonished borrowers to not let the oil level get too low. Should I make a big deal about not over-filling it? What do you think?
Jim Harwick, via e-mail
A: If the engine was only 1 quart over and it still runs fine, it probably is fine. You didn't mention if the oil looked different when you checked the dipstick. If the oil was foamy or frothy, it could be aerated. This can cause oil pressure to drop, which can cause damage.
If the level of oil in the crankcase gets too high, it can reach the crankshaft. The action of the crankshaft will whip the oil, leading to foam and lower oil pressure. Also, if oil levels are too high, it can start to find its way out of the block, through gaskets, seals, or PCV components.
At The Hop
Q: I did a 4/5 drop on my '99 Chevy S-10 shortbed regular-cab pickup. I accomplished the drop with a combination of spindles, springs, and lowering blocks. I added 17-inch Billet Specialties wheels and 40-series BFG rubber. The truck looks great, but now it has rear-wheel hop. Is this normal, and what can I do to eliminate it? Thanks for your help.
John McDonald, Phoenix
A: When you lower a truck, you change the rearend pinion angle. You need to measure and check the pinion angle. A protractor with a movable indicator will allow you to make comparison readings. Make sure the truck is on a level surface when you take your measurements.
Your pinion angle should be 1 to 1-1/2 degrees of downward angle relative to the driveshaft. If this isn't the case, you'll need some leaf-spring mounting shims to achieve the desired angle. Typical shim kits come with 1-, 2-, and 3-degree-angle shims.
Spring wrap-up is another component of wheel hop. Stiffer rear leaf springs will help control hop. A custom spring facility may be able to help you. The right shock absorbers such as adjustable ones can also help. Individually adjustable shocks will let you preload one side of the suspension.