How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 20594
Experience:  BEng hons Mech engineering, in auto industry 22 years
12772297
Type Your UK Car Question Here...
Matt is online now
A new question is answered every 9 seconds

I've been experiencing detonation on a Mitsubishi

Customer Question

I've been experiencing detonation on a Mitsubishi Chariot Grandis, 1999, 170000 kms, 4g64 engine. So far I've checked for intake vacuum leaks (found none), did an engine top chemical cleaning to try and remove excessive carbon deposits and have been using
petrol injector cleaner additives to counter the possibility of clogged injectors inducing a lean mixture. Recently I checked the knock sensor and noticed it has no continuity between either of its pins and the ground. I would like to confirm if the sensor
is indeed supposed to have continuity and if so what kind of ohmage reading I should be getting on a working sensor. I would also be happy to hear about any other items I should check that might be causing this detonation. Many thanks in advance.
Submitted: 1 year ago.
Category: UK Car
Expert:  Matt replied 1 year ago.

hello

unless you have some kind of load resistor then you won't get a resistance reading between the pins

best method to test is to hook up an AC voltmeter and give the sensor a sharp 'tap' with something metallic

you should see a voltage spike on the meter

however what I would say is that these early GDi ( high pressure fuel injection into the cylinder) are very prone to coking up of the inlet manifold and EGR circuit

so I's suggest blocking off the EGR feed into the inlet to see if this helps at all

it would also be worthwhile checking fuel pressure is up to the 3bar mark with a gauge onto the injector rail

Expert:  Matt replied 1 year ago.

Hi

do you still need help?

Bear in mind that the site takes a deposit from you at the beginning andthis is held by the site until you rate my answer at which point the cash is split between the site and the expert. so if my answers have been helpful please rate my answer.

Customer: replied 1 year ago.
Hello Matt, and thank you for your reply.I tested the knock sensor the way you said and got no reading on the AC voltmeter.I also tried measuring the AC voltage while revving the engine (inducing knocking) on several occasions:One time I did get a reading, mostly <5 AC volts, although at some point it spiked to 50 volts. A few minutes later I was getting no reading at all.On all other occasions the voltmeter read zero.Is it supposed to be like this or you think it might be faulty?I haven't checked the EGR circuit yet. I've been horribly busy lately but will do it once I get the opportunity.I'll also try to get a fuel pressure gauge to check the fuel rail pressure. Might take a while though - I live in a remote place in Africa, we don't exactly have this kind of equipment widely available here. Same goes for auto technicians and workshops.A few more details:When the engine is warm and either idling or cruising it starts running rough, with small but sharp rpm fluctuation. It seems like a mild pre-ignition. This started at the same time as the previously mentioned knocking;I am getting no engine trouble codes at all (jump-wired the connector), not even after running the engine with the knock sensor unplugged.What do you make of this?
Customer: replied 1 year ago.
It seems part of my last post got cut out, it should read like this:(...)I also tried measuring the AC voltage while revving the engine (inducing knocking) on several occasions:One time I did get a reading, mostly below 5 AC volts, although at some point it spiked to 50 volts. About 2 minutes later I was getting no reading at all.On all other occasions the voltmeter read zero.(...)
Expert:  Matt replied 1 year ago.

hello

Ok that sounds like the sensor may be working, ideally you need a oscilloscope to spot the changes in voltage but if you're getting a voltage spike when its being tapped then that usually means its fine and you can trust any reading its producing as they usually work or they don't with no in between

the next thing is the why its detonating?

as its either too lean due to clogged injectors or low fuel delivery pressure

or the spark advance is too high but I don't think that there's a distributor on this engine?

or the combustion chamber is badly coked up and so raising the compression ratio of the engine

so fuel pressure check as the next step

Expert:  Matt replied 1 year ago.

Hi

do you still need help?

Bear in mind that the site takes a deposit from you at the beginning andthis is held by the site until you rate my answer at which point the cash issplit between the site and the expert. so if my answers have been helpfulplease rate my answer.