Just when I thought I'd cracked it!!!

We were getting pinging on slow acceleration when in locked-up high gear,but not when on kick-down. We looked and found the following:-

1) a load of crap on the inside where the knock sensor screwed in and was probably blocking it from hearing the pinging

2) another knock sensor! The bloody engine has 2!

Plus we find that a 1994 Optispark gives timing 180deg out of phase, but a 1995 is OK (This is on a June 1994 C4 convertible).

So we replace BOTH knock sensors, new plugs and off we go, until today when in very misty/rainy conditions (well it is the UK) it not only pings on slow acceleration, but also on kick-down.

Current thoughts are:-

optispark?
knock sensor module?
loose wire?

What do you think?

Oh and I forgot to say no pre-ignition codes on the Tech 1

Any help gratefully received.

Thanks

 

The only thought I have is that the '94 Opti isn't out of phase, the spark plug wires are.

Do you have the knock sensor(s) torqued to the exact specs?

Larry
code5coupe

 

I don't know much about the Gen II engines, but it sounds as though the ESC isn't doing it's job. It retards the timing to stop the knock. You should never hear but the briefest period of knock before the ESC stops it.

The "...load of crap on the inside where the knock sensor screwed in" is probably sediment from a poorly maintained cooling system, as the knock sensors replace the block, coolant, drain plugs, from years previous. You didn't switch from the orange, Dex-cool coolant, to the green, ethylene glycol antifreeze, with out a very thorough flush, before hand, did you?

RACE ON!!!

 

"Ping" has got to be one of the most difficult things to correct. I have had GM cars with new, old, rebuilt, with knock sensor and without knock sensors, new exhaust gas recirc valves, retimed etc engines, and all of them have run very well and all of them have "pinged" at one time or another off and on. To some extent I think it might be mixture related, as it has occurred mostly on lighter applications of throttle on part acceleration, when you have a leaner mixture, not on heavier applications of throttle, when the mixture is slightly more rich. Sometimes switching to Mobil gas has worked for awhile. Retarding the ignition till it stops is another way to stop it, but you get a sluggish feeling engine. Sometimes you do nothing and it simply goes away for awhile! Bottom line: My recommendation would be to ignore it unless it is heavy ping. You can also, at times, "drive" through it by the heavier applications of throttle that I described (e.g., getting onto a freeway).

/s/ Chris Kennedy

 

You're absolutely correct that the coolant system had not been well maintained and we have flushed the whole system comprehensively once we found the crap around the knock sensor, before refilling with coolant.

We looked at the re-allocation of the plug leads to suit the '94 optispark but there was clearly something not quite right there.

I also believe that it is the management system not doing its job, a poor connection, moisture on a joint......, who knows?

Is there a better system than the optispark, that seems to have received quite a bit of variable comments, that will easily fit the '94 C4 engine? I am assuming it is a '94, or would the fun and games with the '94 optispark being out of register indicate a '95 or alternative engine having been fitted from the factory (car has only done 35,000)

Best Regards and thanks

 

How long have you had the car and how much of it's history do you know?

How long ago did the problem start and what happened to the car right before that?

Any engine mods? PCM reprogrammed? PCM replaced?

I doubt that the crud in the coolant passage prevented the KS from hearing the knock, since it is the block that transfers the sound, not the coolant.

Your comments about 94-95 optispark phase differences make no sense. Are you saying you tried both 94 and 95 optisparks on the car? Unlikely, since that requires major engine surgery. Are you saying something about the caps? They are interchangeable, so that can't be it. What are you talking about there?

 

LT1 motors have two knock sensors. Either side will send voltage signals to the ECM.

With the Tech-1 connected, hit the left side exhaust manifold and it should show a knock signal. If it doesn't, the sensor may be bad.

You can test for voltage at the sensor while tapping the block near the sensor. Disconnect the sensor and connect a voltmeter between the sensor and a ground. Set the voltmeter for 2V AC and tap on the block near the sensor. If you get a signal at the voltmeter, the ECM may be bad or the chip programming may be bad. If there is no signal, the senor is bad.

When you installed the knock sensors, did you add extra sealant to the threads?? The FSM says not to as the GM parts already have a sealant on the threads. Too much may not allow a sufficient ground for the sensor to work properly.

I would also try a different brand of fuel being careful to make sure that it has a minimum 91 Octane or the equivalent rating in the UK. It's always possible to get bad gasoline or have a delivery tanker dump the wrong fuel into a gas station's tank. Not a common occurance but worth trying a different brand of gas.

 

Many thanks for you help.

The story is an involved one but to simplify...

I have had the car 3 months and am using it regularly, whereas the previous owner only used it at weekends, blasted it and then put it away in garage. What a way to treat a car, let alone a Corvette

The previous owner had a problem with it not running and the optispark was diagnosed as the cause. Rather than buy a new optispark (expensive) he found that from Auto Accesories in the US, he could get a complete tune-up kit with a new 'genuine' optispark for less than the cost of just the optispark. He bought a kit for a 1994 model which the car ostensibly is (28 June 1994). However when fitted it would not run.

My colleague who is very experienced on Corvettes and in whom I have a great deal of faith, looked at the unit, and slowly went through checking ignition, injectors etc, and concluded the ignition rotor was about 180deg out, despite it being the correct one for a 1994.

He spoke to a colleague at RAF Mildenhall (a USAF base in UK) who runs a US car maintenance facility, knows his stuff and almost has a direct line to GM, who said that instead of treating it as a 1994 or 1994.5 car, he would treat it as a 1995 since the build number was a late one and because it sounded like the wrong optispark.

Now here I plead ignorance because I do not know the difference between a 1994 and 1995 optispark, and those of a nervous disposition should shield their eyes....

The master spline on the rotor was machined out and the rotor replaced, 180deg out of phase with its original register. It was a 'firm' fit and was aided with some silicone as well. It will not move! There is no fretting or movement of the rotor to vary the timing. The engine starts and runs fine apart from the pre-ignition problems explained earlier.

So, at the moment we are planning to replace the optispark for the correct (we hope) 1995 one (Did you know it is 180deg different from the 1994? Please confirm if this is the case), getting a new knock sensor module, some platinum plugs and 8mm leads on a shopping trip to the US this December, and anything else you would suggest to crack this one.

The crazy thing is that the pre-ignition disappeared for about 2 weeks and only recently returned in wet/ misty weather. To my mind this suggests a loose/iffy connection or a module that does not like the damp.

Sorry to all you purists if I have got some bits garbled but some of it is based on second hand accounts

As before your helpful comments will be appreciated.

Graham

 

Assuming we are talking about vette parts, then it does not sound like you have had both a 94 and a 95 optispark on the car.

To put a 95 optispark on a 94 car requires replacing the timing cover and camshaft among other things.

Going the other way requires changing the timing cover but not the camshaft.

Neither change involves any "phase difference" AFAIK - people have put 2nd gen cap and rotor kits on 1st gen optis.

The one exception to the above is that the B-body LT1s got the vented optispark in 94, so if the first optispark was ordered for a 94 B-body then that could explain some of the confusion. But that still doesn'
t account for a "phase difference."

I think you need to set eyes and hands on these things yourself or have the mechanic get on here so we can get a first-hand perspective.

 

Fair comment about giving first hand experience rather than second hand via me. A little more light is emerging in the tunnel. It would appear from catalogues that there is an optispark for up to and including 94, a different one for 94/95 and still another one for 95/96. I have yet to bottom this one. And there was I thinking owning a C4 that had been in production for so many years would be straightforward. Stupid child for ever believing that!

 

No. There are only two optisparks. In vettes it is 92-94 and 95-96.

 

WE HAVE A RESULT!!

Despite sounds of pinking (pre-ignition) the Tech 1 was not showing any error codes and in fact was showing that the knock sensing circuits were OK. However when we tapped the block to replicate a knock, nothing was signalled. The knock sensors are new and correctly installed.

So we came to the conclusion that whilst the knock sensors might be telling the knock sensor module, the engine management system was not getting any information and not showing any error codes.

Now we get to the interesting bit.

On the passenger side just in front of the windscreen wash filler is what is described in the Haynes manual for a 92 Corvette as the knock sensor module. It had a part number as follows 10177912. We do not know what this part number equates to, because my car is 94/95 (almost one of the last 94s made we now know) and should not have that unit there. It should have a module in the ECU. But there was no sign of that. So did we have a 92 knock system in a 94/95?

It was not until I looked at the colours for the multiway plugs on the ECU that I noticed it was upside down and once we removed it, the later knock sensor module trap-door was visible. Why was it upside down? Well there are mesh covered holes to ensure no condensation and if the right way up they would allow water in.

Anyrate, the poor old knock sensor module (part 16177690) has been hanging by its clips for 12 odd years and was clearly tired. We took it out, dusted it down and plugged it back in and bingo! One Tech1 that says I hear knocking when it should do. Mystery solved and inexpensively.

Can anyone tell me what 10177912 is and why I still have one, or is it nothing to do with the knock sensor and just using the old bracket.

Thanks for all your help and opinions. very useful. I hope the above narrative is useful to someone else that has a similar problem.

Graham

 

Glad to hear that you got it fixed.

Next time, find a mechanic who knows better than to use a Haynes manual.

My car has nothing in front of the washer fluid fill neck. There is a small electrical box just behind it, but no numbers on that. Is your car by any chance converted to RHD?

The book Corvette Fuel Injection shows the 92-93 knock sensors going straight to the PCM, same as 94-97, so I doubt they have an external knock module. I also think that if they did we'd have seen posts about how to modify it.

The description of 10177912 at gmpartsdirect is "MODULE AS" which is extremely unhelpful...

I found it also described as "MODULO SENSOR DE POSICIONES":
http://web.archive.org/web/200409250...x=46&img.y=11#


Whatever it is, it was $600 a couple of years ago, so don't f with it.

 

I'm glad you found it. And as it can be with a little detective work, the price was right. Maybe it was the first paragraph of post #3.

RACE ON!!!

 

No it was me who (fortunately) had the Haynes manual. My colleague and mechanic has the full GM service manual and I have the Helm one as well. So we had all 3 out on the table investigating this problem.

Actually if it were not for the Haynes showing that there was a intermediate knock sensor module, rather than signals going straight into the ECU, we would not have found the problem quite so easily and would have probably ended up buying a complete ECU!

So whilst I acknowledge that the Haynes manuals are limited, on this occasion it gave the essential clue to look for an intermediate box of tricks.

Thanks for the 'Modulo sensor de Posiciones'. I will now try and find out what this expensive box does. Meanwhile is there no master list of GM part numbers? (preferably in English?)

Many thanks for your consistent help on this one.