Pinking when warm, engine rebuild made it worse

[jonny27]

Hi guys, I have an issue haunting me for a while, I'll try to describe it the best my non-native english can.
So the car is a 1993 Opel/Vauxhall Corsa Sport (it's not really a chav car around these parts but still lynch away ) with the 8v C14SE engine, and I noticed around and year ago a metallic rattle coming from the block after warming up, when accelerating past 2000rpm, especially going uphill. Even though I haven't heard a similar rattle before, I traced it to pinging, since it would stop everytime I filled the tank with premium fuel, and come back filling it with regular 95.
Now, it had 265kKm, some internal wear (I knew it was burning oil), and since it was serving me well I had the engine rebuilt a while ago (couldn't tackle it myself for lack of space and tools), if the pinging was due to old carbon deposits causing hot spots, the rebuild would take care of that, right? If not, well at least it narrows down my options. Instead it completely threw me off course, the rebuild made the pinging worse, it still only pings when warm, but now it does no matter what fuel it's in. Also pings whether at part throttle or WOT, but it seems to me at WOT it only pings farther in the rev range (from 2500-3000 or so).

Idle is stable at both cold and warm, even if a bit low at warm (750rpm when the Haynes manual states 850-900), so it doesn't look like a vacuum leak; spark plugs look fine, are the recommended temperature and don't show the typical black dots from detonation; since it's a C14SE it still doesn't have a knock sensor or EGR, and the DIS is non adjustable (so ignition timing shouldn't be advanced); and I only notice a small loss of power (that would rule out a clogged cat converter heating up the cylinders, right? The loss of power should be a lot more noticeable?). The head was skimmed though, but I find it hard to believe compression would increase to this extent. Any suggestions on what to look at next?

Edited by jonny27 on Wednesday 11th April 11:31

[GreenV8S]

The original symptoms sound right for pinking, but the new symptoms of it happening at part throttle do not. Check for other possible noise sources such as an exhaust header leak or loose part on the engine or exhaust.

[jonny27]

GreenV8S said:

The original symptoms sound right for pinking, but the new symptoms of it happening at part throttle do not. Check for other possible noise sources such as an exhaust header leak or loose part on the engine or exhaust.

Thanks for the input, will do. Not the first time I see mentions of exhaust leaks related to engine rattle (it still sounds like coming from the block), will check if everything is seated properly. As soon as the weather clears up that is (yes, sunny Portugal is not always sunny ).
2 things I forgot to mention by the way. Paperclip test shows no fault codes (wish I could have live data, but Tech1 scanners are almost Impossible to find nowadays), and a curious behaviour possibly related to temperature: after a cold start it only pings about 5-10 minutes after reaching warm operating temperature, but if I turn off the engine after this, and let it sit for a little while (half an hour or so), it will then ping earlier from there, as it reaches warm temperature or even before. I'm tempted to say it's related to something that both heats up and cools down slower than the coolant (the cast iron block itself maybe?), but I see no difference on the coolant temperature than before, the gauge still shows the usual value, so apparently it isn't overheating.

[Sardonicus]

Does the car come with single point/spi or multi point/mpi fuel injection?

[jonny27]

Sardonicus said:

Does the car come with single point/spi or multi point/mpi fuel injection?

MPI. On another forum I have someone suggesting poor fuel pressure actually, but wouldn't a lean mixture cause an higher and unstable idle? I have the exact opposite, idle lower than spec, but stable.

[E-bmw]

A leaning of the fuel air ratio can make pinking worse.

[jonny27]

E-bmw said:

A leaning of the fuel air ratio can make pinking worse.

I know, that's my main suspicion for the pinking itself, but am I missing any cause of lean mixture that wouldn't affect idle? A vacuum leak for instance would make the idle go nuts in my experience.

[E-bmw]

You are correct an induction air leak would cause an erratic/surging idle.

There are other causes of a lean mixture though.

Have you had the injectors out & tested?

Have you had the plugs out recently with the issue happening to see what the ends of them look like, a whitish discolouration rather than light brown may indicate a weak mixture.

[E-bmw]

Also, as the issue could be temperature related has your manual got in it how to test the various temperature sensors for the engine?

Particularly intake & head temperatures.

[jonny27]

E-bmw said:

You are correct an induction air leak would cause an erratic/surging idle.

There are other causes of a lean mixture though.

Have you had the injectors out & tested?

Have you had the plugs out recently with the issue happening to see what the ends of them look like, a whitish discolouration rather than light brown may indicate a weak mixture.

I actually replaced the plugs last week, when I found the old ones were meant for a different series of this engine. They looked slightly brownish, yes, but only helps if I give them a second closer look, might have missed something.
Haven't really thought about the injectors themselves honestly, I'll see if I give them a look when I get the chance.

E-bmw said:

Also, as the issue could be temperature related has your manual got in it how to test the various temperature sensors for the engine?

Particularly intake & head temperatures.

Can't remember honestly, I'll have to give it a look when I get home. I did however try a spare intake temperature sensor, no difference, but I'd really need live data in this case to figure what it is reading exactly.

Edited by jonny27 on Wednesday 11th April 13:55

[TooMany2cvs]

If the head was skimmed as part of the rebuild, that could easily increase the compression enough to make the pinking worse...

IF it really is pinking.

[jonny27]

TooMany2cvs said:

If the head was skimmed as part of the rebuild, that could easily increase the compression enough to make the pinking worse...

IF it really is pinking.

It was, but enough to start pinking on a fuel grade above? Can't say now, but before it had to be pinking, why else would it consistently stop everytime it had premium fuel in? Would trying octane booster in one tank help clear up if it's still pinking or not?

[E-bmw]

Octane boosters don't work, don't bother.

[stevieturbo]

So does better fuel make a difference ?

Is the ignition timing it has currently within manufacturers spec ? Just because it doesnt appear adjustable...doesnt mean it is correct.

Is valve timing correct ?

How does the engine perform aside from this noise ?

Is the engine running normally, ie mixtures when this occurring are normal ?

[jonny27]

stevieturbo said:

So does better fuel make a difference ?

Is the ignition timing it has currently within manufacturers spec ? Just because it doesnt appear adjustable...doesnt mean it is correct.

Is valve timing correct ?

How does the engine perform aside from this noise ?

Is the engine running normally, ie mixtures when this occurring are normal ?

Before the rebuild filling with premium did stop the pinking, it doesn't anymore, it now pinks no matter what fuel it's in.
I honestly didn't think about valve timing, it's not a common cause but can happen, from what I'm finding.
As for how the engine performs, it does feel like it has a slight loss of power, but other than that sounds fine. It doesn't really bog down or anything, just feels it could pick up speed better than it does.

[steveo3002]

might be an idea for a good rolling road to run it up , see whats going on

[jonny27]

steveo3002 said:

might be an idea for a good rolling road to run it up , see whats going on

Not really many around here, but yes a rolling road would possibly be needed to spot it without driving around. I can confirm it does not happen with the car stationary revving it in neutral, only under load.

[steveo3002]

was the cam timing set properly ?

might not conceren your car , but back in the day i worked at vauxhall some had a plug in the engine bay you reversed it for premium /non fuels , not sure if your model has this?

[Sardonicus]

My guess would be an over lean fuel mixture/AFR some of these old cars dont throw fault codes for this condition I also seem to remember coolant temperature sensors being a common failure on that 8v engine

[jonny27]

steveo3002 said:

was the cam timing set properly ?

might not conceren your car , but back in the day i worked at vauxhall some had a plug in the engine bay you reversed it for premium /non fuels , not sure if your model has this?

The RON plug? Already checked it, it's set to 95/regular, and it reads 220ohms as intended. Oddly enough, the 98/premium side is dead, but since it's set to regular, that one side of the plug connects into nothing anyway.
The more I think about it, cam timing could be off indeed, If it suddenly became worse after reassembly, logic says something was not put together properly. It did get a new cam, yes, but it's fixed in a single mount position, no adjustment possible at all. On the other hand, one tooth off would misalign it 7 or 8 degrees maybe?, that is already enough to raise compression.

Sardonicus said:

My guess would be an over lean fuel mixture/AFR some of these old cars dont throw fault codes for this condition I also seem to remember coolant temperature sensors being a common failure on that 8v engine

That's why I wish I had a data tool. But this still isn't OBD2 standard, the only Tech1 reader I know is the Vetronix one, and they usually cost an arm and a leg. Does anyone know of another reader that works with these by the way?

Edited by jonny27 on Thursday 12th April 10:56