I had the 89 out yesterday for a short drive. It detonates at low speed going up small hills. Would the first thing to check be the knock sensor and can I replace it myself or is it complicated enough that I need the dealer to do it. It has 120,000 miles on the engine with new injectors and plugs last year and a 2:59 rear end. Otherwise runs great.
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Re: 89 automatic .
Rick oxygen sensors in your 89 give input to the computer when to enrichen or lean the fuel ratio going through the injectors. My 85 has only one, your 89 might very well have two, my 96 LT4 has two. Oxygen sensors are not to expensive to change out and from what I have read you have a lot of mileage on yours if they have not been changed. The knock sensor is pretty easy to change out as well. Be prepared for loss of coolant because the knock sensors are located on the sides of the block where the block coolant plugs are located on previous Corvettes without knock sensors. A coolant change would be easy with removal of the knock senors. Your knock sensors might have some protective shielding that will have to be removed first for access, but it is a relative easy replacement. Failure of either a oxygen sensor or a knock sensor should initiate a check engine light. -
Re: 89 automatic .
You have an L98, so the setup is likely same as mine and Jim's, both '85's with single knock sensors. The LT1 equipped Corvettes, starting in 1991 began using dual sensors.
If your "check engine" light is not lit, then you have an "intermittent" problem, which will not set a trouble code. There is a manual troubleshooting process in your Service Manual which describes how to isolate the problem. I am loath to changing out parts by sheer guesswork. At first thought, it sounds like it could be the knock sensor, but it might be something else.
JoeComment
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Re: 89 automatic .
No trouble codes yet and highest grade/octane fuel used. Could it be that with the new injectors the timing needs to be adjusted.....Comment
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Re: 89 automatic .
Rick the oxygen sensor sends information to the computer to assist in the pulsing of the injectors. The knock sensor sends information to the ignition system to retard the timing if needed. Don't know about your initial timing, but easy enough to check. I have been using regular no lead in my 85 for over a year now and it runs just fine.Comment
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Re: 89 automatic .
Rick:
It could be a lot of things, and many things will not set a code. One of which is a small vacuum leak, which doesn't tip your A/F ratio too lean. If your leak is on the line feeding your EGR valve, then that will absolutely cause detonation. Check your EGR valve operation, and the vac line feeding it, first. If you don't have the Shop Manual, then get one from:
20 years ago, when I first bought my '85, I decided to try a little experiment to test out these fancy new devices, called "knock sensors", why, I wasn't sure whether they were some sort of contraption to alert someone of the presence of a well endowed woman, or what. Anyway, I took the same ROUND TRIP on two consecutive days with identical ambient temp/humidity patterns. 100% highway driving, speed set identically with the cruise control, no traffic, etc. The first day I filled 'er up with hi-test, and the second day, I used regular. Under normal driving conditions, the car felt the same on both trips, with no detonation. But there was a significant diff in the fuel mileage, due to the fact that the knock sensor was dialing in less spark advance with the regular fuel. When I calculated total cost of fuel, it was actually cheaper to run hi-test. I have been filling up with premium ever since.
JoeComment
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Re: 89 automatic .
Thanks for the help guys.....I've had the shop manual since 1989. Trouble is, without an engine analyzer I'm usually wasting my time troubleshooting anything.I'll check the EGR valve that was just replaced at the end of last year and the line for leaks. I may just go ahead and replace both oxygen and knock sensors as well. I've always used the highest octane I could buy and 130,000 miles later she stills runs fast and strong.Comment
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Re: 89 automatic .
Rick:
Don't throw away money, and don't replace perfectly good original and correct parts! If you do, please send them to me (I will provide you with my mailing address "off the boards"). If you have the Manual, then follow the unit repair section for each suspected component. Follow the specified test procedures for each. In most cases, all that you will need is a VOM, maybe a few jumper wires with alligator clips on the ends, a timing light, tach-and-dwell meter, a paper clip to put the ECM/VCM into field service mode by shorting the A and B terminals, and a straight pin to stick through the insulation or terminal end of a wire to get continuity to your jumper or VOM.
A bad O2 sensor will set a code, because the ECM will not be able to properly control the A/F mixture due to lack of feedback. You will either see a code 44 (lean), or 45 (rich). A bad knock sensor will set code 42 (EST). An outright non functional EGR valve will set code 32, but a marginally functioning one may not. Almost any electrical/electronic malfunction will set a code. A mechanical problem, on the other hand, will not necessarily set a code, and will be manifested primarily as a driveability problem, and/or an "intermittent" problem. As you can probably see, the wonderful electronics on your modern Corvette is not necessarily a panacea, and can sometimes confound even the most experienced mechanics (nowadays, known as technicians).
After 120,000 miles, there is a very slight chance that it could also be a small carbon buildup in one of the chambers i.e hot spot.Comment
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Re: 89 automatic .
Joe,
I very much appreciate the time you are taking to offer assistance. You will be the first in line to receive my perfectly good parts....if I needlessly replace any of them. I've had the 89 since I drove it off the showroom floor of a dearlership in Florida and since that day I would have loved to diagnose the little problems myself.....but alas I'm not comfortable fooling with the electronics. I find the 66 and 67 much easier to work on. You have given me two good leads though....the vacuum leak at the EGR and now the possibility of a carbon buildup hotspot. Those I will checkout first....before I throw any money at it. Thanks again for the ideas.Comment
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