Knock or detination, where do you start?
Thread Starter
Intermediate
Joined: Mar 2002
Posts: 48
Likes: 0
From: Bellflower California
Knock or detination, where do you start?
86" auto 115K Getting the rattle noise when I accelerate moderatly to hard. I use 91 octane w/ outlaw octane boost. It gets worse as the engine reaches operating temp. Should I start with the knock sensor? Where is it? What is a logical progression to work through? :confused:
Melting Slicks
Joined: Apr 2002
Posts: 3,019
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From: down that one road next to that big tree
Re: Knock or detination, where do you start? (JCascia)
fuel pressure should be checked with a gauge taped to the window. drive around & made sure it's not dropping. timing is #2, try 8deg btdc. make sure you unplug your timing wire when you set this. check to make sure your plugs are okay, also make sure your wires aren't leaking and crosstalking. bad fuel injectors also can cause knock from a cylinder going lean. a bad 02 can do it too however most the time that causes a rich condition. use a code reader & see if you have any set failure codes stored also.
Thread Starter
Intermediate
Joined: Mar 2002
Posts: 48
Likes: 0
From: Bellflower California
Re: Knock or detination, where do you start? (gtsyellow)
What is min fuel pressure and what would be desirable. is the factory reg adjustable. should I just go ahead and replace it with an ajustable one to achieve a higher pressure? thanks for the advice :yesnod:
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Knock or detination, where do you start? (JCascia)
Some potential causes are:
-bad knock sensor
-bad fuel
-hot engine
-timing too advanced (check with plugs gapped properly and stock chip)
-too lean
The knock circuit should signal the ECM to pull timing before you hear ping.
The knock sensor is located in the water jacket low on the right rear of the block in front of the starter.
When installing a new knock sensor you may use a little anti-sieze to seal and assure good electrical contact but do not over-torque it.
The other part of that circuit is the ESC module located to the left of the blower motor just above the body pannel with the clam shell open.
-bad knock sensor
-bad fuel
-hot engine
-timing too advanced (check with plugs gapped properly and stock chip)
-too lean
The knock circuit should signal the ECM to pull timing before you hear ping.
The knock sensor is located in the water jacket low on the right rear of the block in front of the starter.
When installing a new knock sensor you may use a little anti-sieze to seal and assure good electrical contact but do not over-torque it.
The other part of that circuit is the ESC module located to the left of the blower motor just above the body pannel with the clam shell open.
Melting Slicks
Joined: Aug 2001
Posts: 2,516
Likes: 0
From: Lockport NY
Re: Knock or detination, where do you start? (65Z01)
The other part of that circuit is the ESC module located to the left of the blower motor just above the body pannel with the clam shell open.
The BIN file in the ECM MemCal got upgraded over the years, too. Spark curves are changed, as are injector pulse widths & timing. Getting the latest MemCal for your car will yield the latest BIN.
If your car ran OK before all this began, then it's a simple mechanical breakdown. Knock sensors are notorious for bad connections. Only one wire; but it breaks down, disintegrates, comes loose, etc. Ecklers has wiring repair kits as well as sensors.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: Knock or detination, where do you start? (JCascia)
What is min fuel pressure and what would be desirable. is the factory reg adjustable. should I just go ahead and replace it with an ajustable one to achieve a higher pressure? thanks for the advice :yesnod:
If the pressure is low check the filter first thing.
Melting Slicks
Joined: Apr 2002
Posts: 3,019
Likes: 0
From: down that one road next to that big tree
Re: Knock or detination, where do you start? (Nathan Plemons)
which brings up another note, fuel filters really should be changed yearly. they cost under $10 and are about as hard to do as changing the oil. you have a clamp and 2 threaded tube incerts to unscrew and your ready to change. i'm sure i could do a complete fuel filter change in under ten minutes, start to finish. i'm actually suprised how many people neglect such a simple yet needed part of maintenance
Burning Brakes
Joined: Feb 2000
Posts: 824
Likes: 2
From: Camden SC
Re: Knock or detination, where do you start? (gtsyellow)
You really want to get this one fixed. The knock sensor (should be located over on passenger side, above the starter, round thing with plastic cover) acts like a little microphone and it is quite possible your's is not picking up the bad vibrations caused by the knock. This device should pick up a knock long before you hear a ping.
Are there any codes?
If you have a volt meter with an OHM setting, check the resistance of the knock sensor. Check your service manual and it may be around 3300-4500 ohms. If the bridge component in the sensor is bad, it will not respond to the vibrations.
Or, just buy a new sensor put it on and see what happens. With 112K, probably wouldn't hurt. I would keep my foot out of it until this issue is resolved.
After the new sensor is installed, as stated above, I would check the base timing.
Hope it's that easy.
dlmeyers 90 coupe zf6 3 speed shocks
Are there any codes?
If you have a volt meter with an OHM setting, check the resistance of the knock sensor. Check your service manual and it may be around 3300-4500 ohms. If the bridge component in the sensor is bad, it will not respond to the vibrations.
Or, just buy a new sensor put it on and see what happens. With 112K, probably wouldn't hurt. I would keep my foot out of it until this issue is resolved.
After the new sensor is installed, as stated above, I would check the base timing.
Hope it's that easy.
dlmeyers 90 coupe zf6 3 speed shocks
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Melting Slicks
Joined: May 2001
Posts: 2,143
Likes: 7
From: San Diego CA
Re: Knock or detination, where do you start? (JCascia)
Stock idle fuel pressure is 42# I think. Also, check for a vacuum leak on the line to the regulator. The vacuum line raises the pressure when you accelerate.
It should be able to hold that pressure for several minutes after you shut down. If it can't, replace it.
It should be able to hold that pressure for several minutes after you shut down. If it can't, replace it.
Burning Brakes
Joined: Sep 1999
Posts: 786
Likes: 31
Cruise-In II Veteran
Re: Knock or detination, where do you start? (JCascia)
This is a situation where a scanner program comes in handy. It will show how the various parts of the engine are interacting together and often pinpoint the problem in seconds. Most of everything else you could try are just shots in the dark hoping that one of the "solutions" solves the problem. Fuel pressure, EGR, ignition problems? Who knows.
Barring using a scanner, are there any codes? They will often give some clues as to the problem but usually won't isolate it to a specific part.
A great scanner program is available for FREE from forum member Craig Moates. Use that with either one of the interfaces he suggests or a cable from AKM and you are in business cheap.
http://moates.net/gmecm/index.html
http://www.akmcables.com/
Barring using a scanner, are there any codes? They will often give some clues as to the problem but usually won't isolate it to a specific part.
A great scanner program is available for FREE from forum member Craig Moates. Use that with either one of the interfaces he suggests or a cable from AKM and you are in business cheap.
http://moates.net/gmecm/index.html
http://www.akmcables.com/
Thread Starter
Intermediate
Joined: Mar 2002
Posts: 48
Likes: 0
From: Bellflower California
Re: Knock or detination, where do you start? (gtsyellow)
The car has had this condition for a while. the fuel pump failed and was replaced along with the filter but this did not change the symptoms.
Melting Slicks
Joined: Apr 2002
Posts: 3,019
Likes: 0
From: down that one road next to that big tree
Re: Knock or detination, where do you start? (JCascia)
also do a leak down test of the engine, if you cracked a ring under detonation the car will do that & there isn't much you can do to cure it short of a rebuild. i remember right before i blew mine up i was thinking, gosh why is this thing knocking with 95oct elf gas? i'm almost positive i had at least one broken top ring. the scan tool idea is also a good one, if you have any set codes you need to know about them & possibly troubleshoot.
Safety Car
Joined: Sep 2001
Posts: 4,137
Likes: 531
From: Glenbeulah Wi
Re: Knock or detination, where do you start? (JCascia)
When my 1986 was stock it also knocked at full throttle and was most pronounced at the shift points. All the sensors were working correctly, my scanner was showing m the knock retard numbers. What I did to make it better but was set the timing at 6 degrees and change the thermostat to a 160 degree unit. After these changes I would only hear the knock at the shift points. I always attritributed the knockto the poor combution chamber design of the iron heads.
