Toyota Truck Pinging Problem

Raxen

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I have a perplexing problem with a 1990 Toyota Pickup with 125K miles on it. This truck is equipped with a carburetor. It has been very well maintained over the years. When the weather starts to become cool and dry as it has been here recently (I live in a rocky mountain state at about 4,200 feet in elevation), this truck will ping incessantly after a cold start as it approaches normal operating temperature. It only happens this time of year (Oct-Nov). Its been happening for at least the last five years and I have not been able to solve this problem. I do not hear any pinging in the winter, spring or summer months, and tuning the engine has had no effect.

I hear the pinging most notably when I start from a stoplight or when I shift into higher gears (third or fourth)and only under light load. If I step hard on the accelerator, I cant hear the pinging. However, if Im driving at moderate RPM, it sounds literally like marbles in a tin can. The odd thing is, after driving it for several miles at normal operating temperature, if I turn it off for a few minutes and then restart it, the pinging is totally gone.

Im thinking three things. First, the combustion temperatures could be excessively high before the thermostat opens. It has a fresh factory 195 degree thermostat in it and I have always used the Toyota red coolant. It has the original water pump, but a new radiator and fairly recent fan clutch. Second, I may have carbon build up in the cylinders causing hot spots. I was going to try a Seafoam or BG 44K treatment to see if that makes a difference. Third, I may have a lazy valve somewhere in the emission system or possibly and EGR valve problem. I was going to change out the vacuum lines (they are original), and I was thinking of pulling the EGR to see if there is any carbon buildup in the tube. Am I on the right track here? Are there other possible things that I should look at?
 

Raxen

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Jim, I'm running mid grade right now which works great most of the time, except this time of year for some reason. I ran the highest grade last year at this time. High octane does minimize the pinging, but it doesn't eliminate it entirely. I'll try the Seafoam to see what effect that may have.
 

csaxon

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Do the gas stations in your area switch to a winter blend of fuel this time of year?
 

Raxen

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That's a really good question. That used to happen here (Utah) at about this time of year, but the requirement for oxygenated fuel was lifted a couple of years ago when air quality improved.
 

2POINTautO

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The requirement may have changed but the actual use of winter fuel is still the norm, ask the station or contact the head company about this issue, have you changed brands where they say they do not use winter fuel. I also vote for SEAFOAM and have you checked the timing vs warm idle speed along with a properly operating advance system. Disabling the EGR is a good step to see if the pinging gets worse and you are on the right track that a restricted EGR will allow some pinging. Does your state have a SMOG inspection or can you get a 5 gas analysis on a Dyno performed? Proper spark plug with proper gap, I am sure you did a good tune up with OEM parts. If nothing seems to help then you could install a set of ONE step cooler spark plug to see if that helps also, good luck.
 

Raxen

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Thanks for the suggestions, 2POINTauto. I will have to check on the fuel. Certainly makes sense that this could be a contributor. Interstingly, as the weather gets colder (Dec, Jan) the problem goes away entirely.

I also wondered about the vacuum advance. I was going to change out all the vac hoses as a starting point to ensure that I was pulling proper vacuum everywhere since they are original. Timing is a little bit advanced on the truck (5 degrees BTDC with line to vac advance plugged--calls for 0 in the factory service manual on the plugged adjustment). However, the truck barely runs at factory spec (tried it several times to be sure) and has absolutley no power. Then again, this truck is not equipped with the High Altitude Compensation (HAC) valve that it is supposed to have above 3,900 feet. Supposedly this optional valve helps to advance the timing when needed to ensure proper air/fuel mixture at altitude. Dealer wants $100 for this part. I may have to consider installing this kit. I never needed it before this pinging issue started though.

Spark plup gap is correct (OEM Denso plugs) and I only use factory ignition parts on this truck. Trying cooler plugs is a good idea.

We do have a smog inspection here. I'd have to look into where to do a 5 gas analysis though.
 

Raxen

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I put Seafoam down the carb, in the tank and in the crankcase tonight according to instructions. I was expecting a bunch of white smoke, but there was none. We'll see if it makes any difference when I drive it in the morning. Idle does seem better, and the responsiveness seems a little crisper.

I wanted to run one other theory by everyone. This truck has a bi-metal vacuum switching valve (BVSV) that is considered part of the EGR system. As I understand it, this valve initiates full operation of the EGR once the coolant warms up. I wonder if this could possibly be defective. Based on the symptoms, maybe this valve is not opening and casuing the EGR system not to work properly? Just a thought.
 

2POINTautO

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As you stated, it a bi-metal vacuum switching valve (BVSV), which means when the engine is cold there should be no vacuum to the EGR diaphram when the engine is cold, even on newer vehicles, there should be no EGR on a cold engine, nor at idle and there is one more case that I cannot remember off of the top of my head, oh, on decel. You mentioned a possible vacuum hose leak and you wanted to change hoses along with this being a good preventive maintenance measure, dont forget that the EGR vacuum diaphram could also be leaking, if you hook up a hand vacuum pump while the engine is idling and pump up the EGR directly, the engine should die, if not and the vacuum pump does not hold a vacuum then the diaphram is leaking, if the gauge holds a vacuum but the engine does not die, then most likely you have a restricted EGR pipe and it needs cleaning or replacing.

All you need to do is hook up a vacuum gauge to the BVSV and monitor it while the engine warms up, if you T into the EGR hose and run a long vacuum hose to the cabin you can probably tell when EGR is activated while cruising in your trouble RPM range, give that a try.
 

Raxen

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Great help! I'll give those things a try. Thanks a million for your input.
 

Paul

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I would suspect the timing chain is slack and causing the trouble.Thats the most expensive fix.Before you delve into that,I can almost guarantee the vacuum line and or the metal pipe that the vacuum line from the egr vacuum modulator to the egr is clogged with carbon,this will cause the pinging every time if its not the timing chain.The metal pipe is hard to clean w/o removing it,and removing it is just as hard if you can't knock out the baked in carbon.You will find that small metal pipe choked with carbon.This clog renders the egr uselss and will cause the engine to ping.I mentioned the timing chain slack because you said this happens with a cold engine.Cold engine gets no egr operation so thats not the trouble.I think once you go thru and give the engine a new timing chain,guides etc,the pinging will be gone.

[attachment deleted by admin]
 

Raxen

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Paul,

To clarify, there is no pinging with a completely cold engine. However, once the engine reaches normal operating temperture after a cold start and a few miles of warm up, the pinging starts. After several miles of driving, it eventually goes away. It definitely could be the timing chain, although it was replaced about 40K miles ago. Probably enough mileage for it to slacken again, however. This certainly would make sense.

Just to be clear, on the EGR are you talking about the small pipe that stems up from the base of the EGR and that connects, via vacuum hose, to the bottom of the vaccum modulator?

I pulled the air cleaner assembly last night and replaced all the vacuum lines. While doing so, I noticed that the large collar nut to the EGR was totally off and hanging halfway down the pipe. I put it back on and tightened it. Pinging, while still present, was less noticable this morning.
 

Raxen

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I didn't notice the picture that you attached to your message until just now. Sounds like we're on the same page regarding which pipe on the EGR might be clogged.
 

Paul

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Ok,Yes,check that vacuum line and the small metal tube for carbon,clean as nessecary.
 

2POINTautO

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Go to http://www.autotechonwheels.com/ , Monthly Tech Tips , Feb 2008 , for an idea of what the inside of the vacuum hose may look like. Click on the first pic and it will expand on its own.
 
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