'94 VW Fox 1.6 AT PINGING

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Ralf_CT
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'94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

I'm sitting with a strange problem on the Fox I've just bought for my mom. It pings terribly under part load (maximum vacuum advance). Thus far I've checked the following:

1. Ignition timing: currently on 6 BTDC, with both vacuum hoses connected (it pings even at TDC). Changing the timing makes no change to the pinging. It has the vacuum advance/retard distributor (retard side connected to the pipe going to the booster, advance to the outlet at the rear of the carb);
2. Tried 95 unleaded and 95 LRP;
3. Checked cam timing, distributor timing and flywheel TDC mark- all marks line up;
4. Had a slight leak at the carb base plate causing hard starting and stalling when cold - changed the plate, engine now purrs, choke can now be pushed in after a minute or so;
5. Checked compression: all around 1,300, one slightly higher;
6. Checked inside the combustion chamber using a mini-camera, found no unusual carbon deposits on the pistons/valves;
7. Plugs currently Champion N7YC;
8. Engine runs between 85 and 90 while driving, rising to 100 (fan kicks in) in traffic. No overheating issues;
9. Spoke to the mechanic who has been servicing the car since 2005. The engine has never been opened, i.e. head not skimmed. Then again it could've been skimmed before 2005, but he reckons never picked up a ping.

The mixture may also be excessively lean, then again it wouldn't purr if that was the case. Performance is fine, no jerking, lagging etc. All I can think of is that the vacuum advance may be over-advancing, then again why would it. I don't have a vacuum pump to check the amount of advance at 'X' mm Hg, however, simply sucking on the pipe produces 33 degrees of advance (then again I'm probably sucking more than the specified 'X' mm Hg).

The 1.6 HM engine is not a high-compression high-performance engine, I don't think the 95 octane is the cause.

Many thanks for any tips which will help me sort this out. It's probably something simple I've overlooked.
Last edited by Ralf_CT on Fri Nov 17, 2017 11:54 pm, edited 1 time in total.
1994 VW Fox 1.6 (HM) AT 138,225km
CaddyJp
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Re: '94 VW Fox 1.6 AT PINGING

Post by CaddyJp »

Ralf_CT wrote:I'm sitting with a strange problem on the Fox I've just bought for my mom. It pings terribly under part load (maximum vacuum advance). Thus far I've checked the following:

1. Ignition timing: currently on 6 BTDC (it pings even at TDC). Changing the timing makes no change to the pinging. It has the vacuum advance/retard distributor (retard side connected to the pipe going to the booster, advance to the outlet at the rear of the carb);
2. Tried 95 unleaded and 95 LRP;
3. Checked cam timing, distributor timing and flywheel TDC mark- all marks line up;
4. Had a slight leak at the carb base plate causing hard starting and stalling when cold - changed the plate, engine now purrs, choke can now be pushed in after a minute or so;
5. Checked compression: all around 1,300, one slightly higher;
6. Checked inside the combustion chamber using a mini-camera, found no unusual carbon deposits on the pistons/valves;
7. Plugs currently Champion N7YC;
8. Engine runs between 85 and 90 while driving, rising to 100 (fan kicks in) in traffic. No overheating issues;
9. Spoke to the mechanic who has been servicing the car since 2005. The engine has never been opened, i.e. head not skimmed. Then again it could've been skimmed before 2005, but he reckons never picked up a ping.

The mixture may also be excessively lean, then again it wouldn't purr if that was the case. Performance is fine, no jerking, lagging etc. All I can think of is that the vacuum advance may be over-advancing, then again why would it. I don't have a vacuum pump to check the amount of advance at 'X' mm Hg, however, simply sucking on the pipe produces 33 degrees of advance (then again I'm probably sucking more than the specified 'X' mm Hg).

The 1.6 HM engine is not a high-compression high-performance engine, I don't think the 95 octane is the cause.

Many thanks for any tips which will help me sort this out. It's probably something simple I've overlooked.
I am taking a huge gamble here with this thought. But is the car not over fueling?

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Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

Isn't pinging usually caused by a weak mixture? Also, gentle acceleration only causes a minimal mixture enrichment, but a large vacuum advance. I was thinking the vacuum pipes may have been connected incorrectly, then again if I connect the vacuum retard line to the carb and the advance line to the booster T-piece I'll have maximum advance at idle, with the timing retarding each time I accelerate.
1994 VW Fox 1.6 (HM) AT 138,225km
Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

While the pinging was previously a 10/10 I've now managed to get it down to a 2/10 by replacing the Champion plugs with NGK BP6ES. I recall having a similar problem with my Peugeot 505GTi some years ago, also remedied by chucking out the Champion plugs and replacing with NGK.
1994 VW Fox 1.6 (HM) AT 138,225km
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Re: '94 VW Fox 1.6 AT PINGING

Post by panic-mechanic »

Pinging like that is sometimes caused by a head gasket that is blown between two cylinders. The higher than other compresion reading might be pointing you to that.
Yes lean mixture rather than overfuelling could cause detonation.
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Re: '94 VW Fox 1.6 AT PINGING

Post by VAG Fan »

I'm a bit surprised that your HM has the advance-and-retard mechanism. I've seen it before (on my old 1983 Golf 1 GTS, FR engine), but my 1990 Fox (HM) has a single vacuum line on the (OEM) advance mechanism. Carburetor is the OEM Keihin 26/30DC.

Which carburetor are you using on the Fox?

I also seem to remember that at least on some carburetors, timing should be set without the vacuum advance connected? But I might be mixing up different carburetors.

That being said, the one time I went to the coast with my Fox, I also got pinging at light throttle around 2000 rpm, in spite of using 95 octane. And as I later found out, it did have a slightly blown head gasket, as Stephan suggested (although, in my case, it was mostly losing compression into the cooling jacket).
Mark R.
- - - - - - - - - -
2004 VW Golf TDI, type 1J, AHF, 317k (2016-current) --- daily
1990 VW Fox, type 16, HM, 304k (2005-current) --- spare runaround
Previous:
1992 Audi 500 SE, type C4, AAR (2001-2020) --- nice while it lasted
1983 VW Golf GTS, type 17, FR (1992-2005) --- most fun car I've ever had
1978 Audi 100 GLS 5E, type 43, WC (1991-92) --- died in the side of a Rekord who cut me off
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Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

I've managed to sort out the pinging: replaced the leaking carburettor base plate (didn't reduce the pinging but cold starts and idling are now 100%, choke can be pushed in after 30s), fitted NGKs and retarded the timing a touch more. Idling and performance still 100%. No coolant loss/contamination, no overheating, no smoking - head gasket fine. Carburettor standard OEM Keihin with A/C (although the A/C and temp gauge work intermittently - I rely on an additional aftermarket gauge fitted by the previous owner, runs at around 90 degrees, rising to 100 in traffic when the fan kicks in).
1994 VW Fox 1.6 (HM) AT 138,225km
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Re: '94 VW Fox 1.6 AT PINGING

Post by panic-mechanic »

you can have a blown gasket between the thinnest sections of the wall between the sleeves and have no coolant loss or contamintion , no overheating no smoking so if it continues or gets worse don't think the things you mentioned is the only thing that indicates a head gasket fail.
Stephan van Tonder - Jhb - Putfontein Benoni
'05 Audi A6 3.0L TDI Avant
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Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

panic-mechanic wrote:you can have a blown gasket between the thinnest sections of the wall between the sleeves and have no coolant loss or contamintion , no overheating no smoking so if it continues or gets worse don't think the things you mentioned is the only thing that indicates a head gasket fail.
Will keep an eye on it. :thumbup:
1994 VW Fox 1.6 (HM) AT 138,225km
Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

panic-mechanic wrote:Pinging like that is sometimes caused by a head gasket that is blown between two cylinders. The higher than other compression reading might be pointing you to that.
How would a slightly higher compression in one cylinder indicate a leaking head gasket between two cylinders? If compression is leaking between cylinders, those two cylinders should have a lower compression than the rest.
1994 VW Fox 1.6 (HM) AT 138,225km
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Re: '94 VW Fox 1.6 AT PINGING

Post by VAG Fan »

Think about it. On firing order 1-3-4-2, just after cylinder 3 has fired, it is being driven down through its work stroke (by hot gases at several tens of bars) while cylinder 4 is going up through its compression stroke. Pressure in cylinder 3 is higher than in cylinder 4, at least for most of that stroke. So if there is a leak between the two, cylinder 4 is charged with extra pressure (and heat!) from cylinder 3. By the time no. 4 fires, it might well be charged over its design spec and detonate. The same goes for cylinders 2 (work) and 1 (compression).

So, I think it's a feasible concept, at least on the bridges between 1 and 2, and 3 and 4.

However, I must add that the pinging I heard on my Fox was not only one cylinder, but sounded like all of them.
Mark R.
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2004 VW Golf TDI, type 1J, AHF, 317k (2016-current) --- daily
1990 VW Fox, type 16, HM, 304k (2005-current) --- spare runaround
Previous:
1992 Audi 500 SE, type C4, AAR (2001-2020) --- nice while it lasted
1983 VW Golf GTS, type 17, FR (1992-2005) --- most fun car I've ever had
1978 Audi 100 GLS 5E, type 43, WC (1991-92) --- died in the side of a Rekord who cut me off
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Re: '94 VW Fox 1.6 AT PINGING

Post by VAG Fan »

P.S.: whether this argumentation would still hold during non-firing compression tests, I can't say.
Mark R.
- - - - - - - - - -
2004 VW Golf TDI, type 1J, AHF, 317k (2016-current) --- daily
1990 VW Fox, type 16, HM, 304k (2005-current) --- spare runaround
Previous:
1992 Audi 500 SE, type C4, AAR (2001-2020) --- nice while it lasted
1983 VW Golf GTS, type 17, FR (1992-2005) --- most fun car I've ever had
1978 Audi 100 GLS 5E, type 43, WC (1991-92) --- died in the side of a Rekord who cut me off
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Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

VAG Fan wrote:P.S.: whether this argumentation would still hold during non-firing compression tests, I can't say.
You raise a valid point which I also considered, however, if 3 was topping up 4's compression, which is next in the firing cycle, why would the compressions on 1,2,and 3 then be equal? If there was a leak between 3 and 4 then 3 should have a lower compression than 1 and 2, not so?
1994 VW Fox 1.6 (HM) AT 138,225km
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Re: '94 VW Fox 1.6 AT PINGING

Post by VAG Fan »

Yes, as far as my rusty brain can see, it should.
Mark R.
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2004 VW Golf TDI, type 1J, AHF, 317k (2016-current) --- daily
1990 VW Fox, type 16, HM, 304k (2005-current) --- spare runaround
Previous:
1992 Audi 500 SE, type C4, AAR (2001-2020) --- nice while it lasted
1983 VW Golf GTS, type 17, FR (1992-2005) --- most fun car I've ever had
1978 Audi 100 GLS 5E, type 43, WC (1991-92) --- died in the side of a Rekord who cut me off
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Re: '94 VW Fox 1.6 AT PINGING

Post by panic-mechanic »

Well believe or calculate what you want. At least about 4 times in my lifetime I had a pinging car that could not be resolved by timing retard and octane up and whatever and at the end of the day there was a head gasket blown between cylinders.
So once you have exhaust eveyrthing else and if it is still there or gets worse - whip the head off and have a look.
I can only tell you what I have seen in 35 years of fiddling with cars.
Stephan van Tonder - Jhb - Putfontein Benoni
'05 Audi A6 3.0L TDI Avant
'09 Touareg 3l TDI
'13 VW CC 2l tdi (repair project)
'05 Touareg v10

Perfect Power dealer. I do dyno tuning.
Ralf_CT
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Re: '94 VW Fox 1.6 AT PINGING

Post by Ralf_CT »

panic-mechanic wrote:Well believe or calculate what you want. At least about 4 times in my lifetime I had a pinging car that could not be resolved by timing retard and octane up and whatever and at the end of the day there was a head gasket blown between cylinders.
So once you have exhaust eveyrthing else and if it is still there or gets worse - whip the head off and have a look.
I can only tell you what I have seen in 35 years of fiddling with cars.
The pinging is 99.999% gone. I'm happy, but will take your advice and keep an eye on it. :smile:
1994 VW Fox 1.6 (HM) AT 138,225km
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