HT4100 4.1, 4.5, 4.9 Discussion, 4.9L '92 Knocking...or missing, help! in Cadillac Engine Discussion; Hmm, that computer thing sounds like it may be the problem...if it were a bad injector, it would be bad ...
My mechanic thinks my engine is healthy, but was wondering about my CV joints. Could they be causing the rumbling at 50+?
Rumble could be caused by CV joints or hub bearings, or both. In either case, you need to check the CV joints for the obvious, torn boots, dry, dirty, or worn links. Firmly grasp the tire with your hands at 12 and 6, then at 3 and 9, shake the front wheels when the car is off the ground. There should be no noise or movement. With front wheels off the ground, (if only the front is raised, be sure to block the rear wheels) place the gear selector in neutral, and turn the wheels by hand, left, and right, forward and reverse. Again, there should be no noise.
Hmm, that computer thing sounds like it may be the problem...if it were a bad injector, it would be bad ALL the time, not just at certain times, right?
Think I could get a new ECM, and if it didn't fix it, I could return it?
No, and No. A bad injector if misfiring some or all of the time may not be evident unless you know what to look for. The primary indication is reduced fuel mileage. Over feed is evidenced by black carbon deposits and poor low speed performance, under feed is evidenced by elevated coolant temperatures, and poor performance. Often times electronic parts are not refundable. Check before you buy.
The procedure to visually check all injectors at the same is outlined several times throughout this forum. It would not make sense for me to rewrite it. There is also a diagnostic procedure to check each individual injector by way of the CCDIC, also outilined several times in this forum. Other possible fuel related problems are FPR (Fuel Pressure Regulator) and/or fuel pump. The fuel pump is the least likely of all culprits, but should not be ignored, and tested for correct pressure under normal driving contitions.
A good running older 4.9L Cadillac engine should average 15/16 MPG in city and 22/24 MPG highway driving. These numbers are not cut in stone, but are average for a 13 year old Cadillac with proper maintenance. More is better, less suggest there may be a problem.
A basic tune consists of new plugs, new wires, new cap, new rotor all of the same quality AC Delco. Do not cheap out anywhere here. Do not spring for the dual platinum plugs, buy ONLY what Cadillac specifies in the owner manual. You would be wise to do the basic tune before attempting any other diagnostic. NEVER allow a wire to drape over or lay near an exhaust manifold. This sounds simple, but I have seen this cause big problems more than once and is evidenced by all old problems gone after the tune, but soon to return. As always, disconnect the negative battery cable BEFORE you begin the basic tune.