Detonation

[wesley]

HELP -My 1997 C5 has random detonation- Car runs perfectly except occasionally engine will start to knock loudly at low speed and idle -
It sounds scary -like rod bearings rattling. Engine light illuminates and OBD scanner displays P108 MAP sensor high input -
After resetting , using the OBD scanner the car runs fine again when restarted .
Have checked MAP sensor, wiring, connectors grounds, and vacuum lines but everything seems OK- Any ideas?
Wesley

 

[timfitz63]

HELP -My 1997 C5 has random detonation- Car runs perfectly except occasionally engine will start to knock loudly at low speed and idle -
It sounds scary -like rod bearings rattling. Engine light illuminates and OBD scanner displays P108 MAP sensor high input -
After resetting , using the OBD scanner the car runs fine again when restarted .
Have checked MAP sensor, wiring, connectors grounds, and vacuum lines but everything seems OK- Any ideas?
Wesley

Since you specifically didn't mention it, I'll ask: you're not using lower octane/cheap (i.e., not Top Tier) fuel, are you?

 

[dadaroo]

Here is a link the that DTC.

https://www.dropbox.com/s/cqzrg9dlhn3ttlp/1997 DTC P0108.pdf?dl=0

Did you know you can pull codes from your dash in the C5? It will pull all 325 codes from the 11 or 12 modules and not just the emissions related ones that most readers do.


https://www.youtube.com/watch?v=SviRjIsy9G4&list=PL7AcRmt53xvQVwDAGikaqYGw6ZivWB-2K


READING YOUR C5 Engine Diagnostic Trouble Codes (DTC)

This procedure should be carried out any time you experience a problem with your C5. Most inexpensive store bought aftermarket code readers will ONLY read power train DTC’s. Reading the DTC’s with the C5 built in code reader will allow you to read ALL the modules in the vehicle.

The Diagnostic Display Mode is entered with the following procedure:
1) Turn on the ignition but don't start the engine.
2) Press the RESET button to turn off any warning messages. (i.e. door open, trunk open ect)
3) Press and hold the OPTIONS button
4) While holding OPTIONS, press FUEL button four times within a 10 -second period.

Initially, the on-board diagnostics go into an Automatic Mode which will cycle through each module and shows diagnostic codes in a pre-set sequence: PCM - TCS - RTD - BCM - IPC - RADIO - HVAC - LDCM - RDCM - SCM - RFA. All codes will be displayed for each module. ( i.e. PCM = 4 codes) If none are present in a given module, you will see No More Codes on the display.

There are two types of diagnostic codes, Current and History designated with a letter suffix, “C” or “H”. A current code indicates a malfunction is present in the module displaying data. A history code indicates a problem existed sometime in the last 40 or 50 ignition cycles. When not accompanied by a current code of the same number, it's potential evidence of a previous problem, now resolved, that was not removed by clearing the codes. More likely it's an indication of an intermittent malfunction.

Intermittent codes are the most challenging of the diagnostics. An intermittent code may have happened once, may have happened more than once but is inconsistent or may be happening on a regular basis but not at the time the codes are displayed. History codes can also be caused by a current malfunction in a system that is not operating at the time codes are displayed. An example is the rear window defogger which doesn't operate until the Body Control Module detects engine rpm. For history codes set by a module that does not operate with the key on and engine off, a special diagnostic tool called a Scan Tester is necessary to properly diagnose the malfunction.

Once the system has displayed all modules, it goes into the manual mode which allows selection of each module using combinations of Driver Information Center buttons. Manual mode can also be entered during the automatic sequence by pressing any button except E/M. Once the display shows Manual Diagnostics, select a module by pressing the OPTIONS button to go forward or the TRIP button to go back. Once a module is selected, a code is displayed, and if more than one are present; press GAGES to go forward or FUEL to go back.
To exit the diagnostic mode at any time, press E/M. If you want to erase codes in a given module, press RESET To reset the codes once in manual mode, press and hold RESET until it displays NO CODES Press OPTIONS to go to the next module. Repeat the steps until you have reset the codes in all the computer modules.
NOTE!! Only reset the codes IF you want to - it is NOT necessary to do this. Clearing a code does not repair a problem. You are simply erasing the evidence of it in the module's memory. If you clear the code/s, and extinguish the Check Engine Light, your emissions status ready will NOT allow you to pass an emissions test until you have completed the required driving cycles. There are a few body module DTC’s that if set will prevent the module from operating properly. Once the DTC is cleared, the module will return to full function. This is not true for power train DTCs.

If you have never read and cleared your codes, there will probably be a lot of old history DTCs. It is recommended that you clear your codes and see if any come back during a driving cycle. Those are the ones that you need to concentrate on diagnosing.

Once you have the codes, the next question is: What to do with the information?
First, consult the factory service manual. Any serious C5 Do-It-Yourself owner should invest in the Corvette Service Manual of the appropriate model year. The Service Manual is really a requirement if you want to understand and work on your C5.

NOTE and a WARNING. You can read the DTCs while the engine is running. I pull mine up all the time while driving.
WARNING. Don’t become distracted while reading DTCs while your driving and cause an accident!!!!! Use common sense and drive safe.

These are some very good C5 Diagnostic Trouble Code (DTC) explanation web sites!!! They also explain how to read the DTCs

Here are some very good sites that explain what DTC mean:

http://www.gearchatter.com/viewtopic11755.php

http://www.obd-codes.com/trouble_cod...d-ii-codes.php

Make sure to include the H or C suffix when you post your DTCs!!

NOTE!!! Start the car and let it idle. Then take it for a drive and cause the problem to occur. When you return home pull the codes BEFORE you shut off the engine.


Do NOT discount the codes with the H following them. They can provide an audit trail for issues going on with the car. They ARE important in diagnosing problems.

Post ALL the codes that you pull with the engine running..........

Some codes go away WITHOUT setting an H code when you shut the engine off. I suspect you will have several codes that will show C or HC that do NOT show when the engine is turned off.


Mr. Sam

 

[Vettehead Mikey]

Nothing in the OP's description sounds like detonation. Low octane gas will not give the symptoms listed nor would non top tier gas.

 

[timfitz63]

Nothing in the OP's description sounds like detonation. Low octane gas will not give the symptoms listed nor would non top tier gas.

OP describes it as such; he hears knocking, albeit at idle. Just following Sherlock Holmes' method: eliminate what it isn't, and what's left is the answer...

Let's let him answer the questions put to him instead of opining on his behalf...

 

[Vettehead Mikey]

Just following Sherlock Holmes' method: eliminate what it isn't, and what's left is the answer...

This is exactly the process followed. Detonation cannot occur at idle/low speed conditions (particularly on computer controlled cars) without other correlating faults at other speeds or loads. No engine will detonate at idle yet not under other conditions if fuel octane rating or if amount/type of additives is a factor. The fault code observed has no connection to any function that would induce detonation.

Ergo, detonation can be eliminated as a factor.

 

[Hib Halverson]

HELP -My 1997 C5 has random detonation- Car runs perfectly except occasionally engine will start to knock loudly at low speed and idle -
It sounds scary -like rod bearings rattling. Engine light illuminates and OBD scanner displays P108 MAP sensor high input -
After resetting , using the OBD scanner the car runs fine again when restarted .
Have checked MAP sensor, wiring, connectors grounds, and vacuum lines but everything seems OK- Any ideas?
Wesley

You sure this is not engine mechanical noise and not detonation or "spark knock"?

Next time that code sets, connect your scanner and see what the failure records read, especially, if there's any knock retard.

Lastly, when you clear the code, does it reset? If so, how quickly does it reset.

When the knocking is going on and that engine light is one, is the engine idliing smoothly or misfiring or surging?

 

[kpic]

Is it a ticking noise; that could be a lifter.

Any mechanic's ability to diagnose by sound only works when he can hear the noise. Not being there to hear this "knock" we run into interpretation of a knock is a "tick". And of course, to others, a tick is a knock

What we know is spark knock is caused by gasoline with too low an octane rating. It sounds like a pinging when accelerating,

A tapping from the top of the engine could be a problem some place in the valve train.

If the noise goes away at higher RPMs could be due to higher oil pressure. Or depending on what is the original cause, other noises could be blocking it out.

A knocking at the bottom of the engine could be a worn bearing on the crankshaft.

There are 4 could be guesses.

 

[wesley]

Thanks for the suggestions guys.
The car runs perfectly silky smooth most of the time with normal oil pressure/ temperature etc.
It has the normal LS1 valve gear light tapping which disappears 10-20 seconds after starting.
After a couple of miles of smooth (30 mph ) running , sometimes the knocking will start abruptly - It sounds just like low octane fuel in a high compression engine- or the "running on" / detonation noise, sometimes heard when shutting off overheated older V8 engines.
When the car is turned off and restarted it is fine again-
Anyway I'm thinking of maybe an intermittent electronic timing fault - I will keep checking with the OBD scanner etc- I was hoping someone had experienced a similar symptoms with a chevy v8.

Wesley