c4 rattle
05-19-2021, 08:42 PM
#1
6th Gear
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c4 rattle
Hello I am new to this and have some mechanical knowledge, but currently I am trying to gain more so take it easy on me please lol. So I have a 1985 corvette that I just bought. When I go to accelerate hard it makes a rattle sound around the driver side front tire. It seems to mostly do it when the car is warmed up, but sometimes I'll start it up and get on the gas hard but nothing rattles the whole drive so its kind of a hit or miss with it. Any suggestions as to what it could be? Ive thought that maybe something could be loose, but when it starts rattling it loses power/acceleration.
Any help greatly appreciated!!
Any help greatly appreciated!!
05-19-2021, 09:09 PM
#3
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05-19-2021, 09:49 PM
#4
Safety Car
Most likely it is detonation, are you running super unleaded fuel? 1985 had a tendency to detonate and as they age this gets worse. The first thing i would do is make sure it is mechanically up to snuff, i other words a tune up. Also make sure the base timing is set to 6 degrees. After that see how it runs and report back.
05-19-2021, 11:06 PM
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Most likely it is detonation, are you running super unleaded fuel? 1985 had a tendency to detonate and as they age this gets worse. The first thing i would do is make sure it is mechanically up to snuff, i other words a tune up. Also make sure the base timing is set to 6 degrees. After that see how it runs and report back.
05-20-2021, 07:28 AM
#6
Safety Car
05-20-2021, 04:50 PM
#7
Drifting
welcome to the forum....85's do better with premium...however, making sure you're within specs is necessary....timing, tps, etc. a GM factory manual is essential.....good luck....lots of help here, just always be specific and and mention year.....
05-20-2021, 08:01 PM
#8
Safety Car
When the 85s were validated, 98 Octane Premium and 94 Octane Regular were the commonly available grades. So yeah, 35 years later the best available grade isn't as good as the crap-grade was at the time of development.
I'm an OO of an 85. I don't drive it much, but it doesn't detonate at WOT on the currently available Premium which I think is 91 in SoCal. I suspect something else is up with the OP's 85. Base timing is wrong, combustion chamber or piston top deposits are raising compression, or an intermittent knock sensor or amplifier problem. There is a DTC Code 43 for the ESC. Each time the engine is started, then crosses a certain Coolant Temp threshold (so usually only the first start of the day) the ECM will command spark advance until knock is detected, then return the timing to normal. This is the self-test, and if it fails, Code 43 is set. In my 85, the self-test usually results in 3-4 gentle "knocks", then it doesn't do it again for the remainder of the drive. It is audible, and I am accustomed to hearing it under the conditions in which it occurs. If you don't have Code 43, the ESC system is functioning.
That the OP is hearing detonation, accompanied by a "loss of power" indicates the detonation is pretty severe. Figure out the cause and fix it before engine damage occurs. When you hear knock with a loss of power as well, get off the throttle. Don't blow it up.
Pull down the ECM and open the little compartment and look at the PROM. If you find a Super Chip or Jet chip, that is your problem. These old-school calibrations have aggressive spark timing which are unsuitable for today's commonly available pump fuels. Take it out. It should be the stock PROM piggy-backed onto a holder that plugs into the ECM slot. Remove the OEM PROM, and plug it directly into the ECM slot.
I'm an OO of an 85. I don't drive it much, but it doesn't detonate at WOT on the currently available Premium which I think is 91 in SoCal. I suspect something else is up with the OP's 85. Base timing is wrong, combustion chamber or piston top deposits are raising compression, or an intermittent knock sensor or amplifier problem. There is a DTC Code 43 for the ESC. Each time the engine is started, then crosses a certain Coolant Temp threshold (so usually only the first start of the day) the ECM will command spark advance until knock is detected, then return the timing to normal. This is the self-test, and if it fails, Code 43 is set. In my 85, the self-test usually results in 3-4 gentle "knocks", then it doesn't do it again for the remainder of the drive. It is audible, and I am accustomed to hearing it under the conditions in which it occurs. If you don't have Code 43, the ESC system is functioning.
That the OP is hearing detonation, accompanied by a "loss of power" indicates the detonation is pretty severe. Figure out the cause and fix it before engine damage occurs. When you hear knock with a loss of power as well, get off the throttle. Don't blow it up.
Pull down the ECM and open the little compartment and look at the PROM. If you find a Super Chip or Jet chip, that is your problem. These old-school calibrations have aggressive spark timing which are unsuitable for today's commonly available pump fuels. Take it out. It should be the stock PROM piggy-backed onto a holder that plugs into the ECM slot. Remove the OEM PROM, and plug it directly into the ECM slot.
05-22-2021, 02:18 PM
#9
Safety Car
Another common cause of "Detonation" or "Pinging" is when a Car is being driven with a defective EGR (Exhaust Gas Re-circulation) system. When the EGR fails it causes the Nox numbers go very high. This is occurring due to higher Combustion Chamber temperatures. Watch the EGR while you rev the engine and you should see move up or down a bit. During the summer these issues like to pop up as the weather starts heating up.
The EGR on my 1988 C4 failed and I learned about this after failing an emissions test. It was a major PIA to get the carbon and junk out off the lines as it was hard as a rock in the lines. On the later C4's they had a Vacuum Solenoid that operated the EGR with a Pulse Width Modulation signal being sent to operate the Solenoid. I had a bad EGR Vacuum solenoid which was very hard to find when I had to do my EGR. I ended up with a much bigger job than I had planned on but I did a whole lot more to the Corvette. I found a leaking Injector, bad vacuum lines and dirty connections everywhere I looked it seemed. The Corvette ran much better after I replaced the EGR.
Due to the cost of parts and labor there are people who are choosing to disable or delete their EGR versus Fixing it. To disable it you have to have it "removed" from the code to get the most out of your engine. Simply unplugging the EGR Solenoid does not disable it and your troubles with detonation will continue. I just keep mine operational as I like to drive the Corvettes as daily drivers when I can.
The EGR on my 1988 C4 failed and I learned about this after failing an emissions test. It was a major PIA to get the carbon and junk out off the lines as it was hard as a rock in the lines. On the later C4's they had a Vacuum Solenoid that operated the EGR with a Pulse Width Modulation signal being sent to operate the Solenoid. I had a bad EGR Vacuum solenoid which was very hard to find when I had to do my EGR. I ended up with a much bigger job than I had planned on but I did a whole lot more to the Corvette. I found a leaking Injector, bad vacuum lines and dirty connections everywhere I looked it seemed. The Corvette ran much better after I replaced the EGR.
Due to the cost of parts and labor there are people who are choosing to disable or delete their EGR versus Fixing it. To disable it you have to have it "removed" from the code to get the most out of your engine. Simply unplugging the EGR Solenoid does not disable it and your troubles with detonation will continue. I just keep mine operational as I like to drive the Corvettes as daily drivers when I can.
05-23-2021, 11:18 AM
#10
Instructor
I would check to make sure your ESC is plugged in. It is mounted near the blower motor on the passenger side of the engine bay. It should be stepping in to retard timing before you can hear detonation. You also may have a defective knock sensor.
When I bought my 85 the ESC was unplugged and the car would ping under heavy throttle when hot. The previous owner unplugged it because it was not wired correctly and would constantly retard the ignition timing. After securing the loose ground wires to the ESC it worked perfect. No more detonation or significant timing retard under heavy throttle. I also run 89 octane to be safe. I think 91/93 octane is probably unnecessary on a bone stock 85 but if it makes you feel comfortable go for it.
When I bought my 85 the ESC was unplugged and the car would ping under heavy throttle when hot. The previous owner unplugged it because it was not wired correctly and would constantly retard the ignition timing. After securing the loose ground wires to the ESC it worked perfect. No more detonation or significant timing retard under heavy throttle. I also run 89 octane to be safe. I think 91/93 octane is probably unnecessary on a bone stock 85 but if it makes you feel comfortable go for it.
05-23-2021, 01:00 PM
#11
Safety Car
That is a great idea to verify that your knock control system is working properly. A simple way to find out if it is working is to open the hood and have the engine running. Then Using a metal wrench just tap on the exposed areas of your cylinder head. The idle should change (very briefly) IF your knock sensor is functioning. I thought the L98 would retard the timing when it doesn't see a functioning Knock Sensor. I am pretty sure they set the default it that way to protect the engine.
It is a very important subsystem in the engine operations and important to have functional. The Knock Sensor is effectively a Microphone that listens for anything to make an "unusual" noise or bang. A loose bracket could make a noise that can be mistaken as a Knock and causing the timing to retard a bit.
It is a very important subsystem in the engine operations and important to have functional. The Knock Sensor is effectively a Microphone that listens for anything to make an "unusual" noise or bang. A loose bracket could make a noise that can be mistaken as a Knock and causing the timing to retard a bit.
05-24-2021, 10:15 PM
#12
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UPDATE: So I switched it to premium, which made it run a little better for the first 2 or 3 days rough starts but thats gonna be unavoidable when changing the gas. Tune up was not done yet. Today I went to Walmart and everything was fine got on the interstate for a little bit and got off at the red light. after the light I went to takeoff and it sputtered and acted like it was going to stall. drove it the rest of the way home, couple hours later went for a test drive to see if it was still doing it and now on every takeoff it sputters and I get no horsepower for about 3-4 seconds of pressing the gas pedal. any ideas as to what could've happened or what to do other than the tune up (coming within the next week or two)?