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Discussion Starter #1
Does anyone remember the old Wankel powered snowmobiles? How they kinda went BRIIP BRIIP BRIPP when they were idling or running at low throttle.

Well,, when a 4.3 sounds like that it's NOT a good thing.

Here's the symptoms. Engine starts fine and runs well cold. As it warms up it picks up that Wankel sound and gradually develops a skip that can be quite pronounced. Until it gets really bad it is only apparent at very low throttle settings. A couple times it has gotten to skipping so bad it can barely accelerate.

Let's say you're climbing a hill, plenty of power and running smooth. Hit the top and back off the throttle and the skip starts. Start downhill and it gets worse as the engine starts holding the truck back.

Or traveling on a level road at just above the speed for the 3-4 shift. Just a slight stumble occasionally until the shift, then, as soon as OD engages it starts to stumble. Feed in a little throttle and get it back into 3 and it smooths out.

When it's not skipping and you nail the throttle it pulls fine and maintains power right to the transmission shift points. From that I tend to discredit the fule filter as a problem, if it is flowing enough to reach high RPM it should be flowing enough to carry cruise throttle at 40 mph.

So far, more in the idea of routine maint. rather than diagnosis, it has gotten new plugs, wires, cap and rotor. Still stumbling.

Last week we had a no start after tinkering with and finally replacing the starter. (This was after the ignition work had been done). We finally hooked up a new coil and it lit right off. At that point we assumed we had flooded it by repeatedly cycling the fule pump with the engine not cranking.

Once it had started we swapped the wires back onto the old coil and it took off fine, but my helper did mention it looked like a pretty yellow spark off the old coil. I'm wondering if it's worth throwing a coil at it at this point.

If I'm right here, when the computer sees a low load, part throttle condition it advances the timing. As throttle is opened the computer pulls off timing to avoid spark knock at high loads. It that is the case, could a weak coil not be putting out enough voltage to light the fire under a low throttle, high advance condition?
 

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Alan

You cannot flood a fuel injected engine by cycling the key.It will run the fuel pump for a few seconds,but no fuel is injected until the computer realizes the engine is cranking or turning over.The only time it will flood,is if the injectors are leaking.You can cycle the key on and off a few times,and watch for fuel dribbling from the bottom of the injectors and into the throttle bores.


It sounds to me like you have a lean miss.Under light throttle in OD,you have a heavy load placed on the engine due to the reduced gear ratio in OD.It also runs at a much lower RPM.This is the hardest mixture to fire,so if you have a secondary ignition problem,it will show up most under these conditions.


I'd try the new coil,as i have seen a few do that.You can test it by spraying it with water from a squirt bottle and see if it starts to miss,or you see visable arcing.Run a grounded test light around the coil to check for spark leakage.

If it's not that,then I'd be looking for a lean fuel condition.Change the fuel filter,and check the fuel pressure.It may be a plugged filter,or a weak pump.It could also be a partially plugged injector too.You can clip a timing light onto the coil wire and aim it at the injectors.Look for a clean,consistant spray pattern,which should increase when you crack the throttle.

If that stuff check out,you need to get into the computer to look at the data stream.It could be a lazy oxygen sensor causing a lean condition.You can jumper the A and B terminal in the ALDL connector under the dash,and check it for codes too.You will need to find a good scanner to get into the computer and look at oxygen sensor values,fuel trims,and advance.


You are pretty much correct about the spark timing.They use the computer,an EST (electronic spark timing) module and the knock sensor to control timing.The computer bases timing on engine inputs and spark curves,and uses the knock sensor to "listen" for detonation.If detonation is detected,it retards the timing.

Let us know what all you find.
 

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That was some reply Chris! :D Stop over my house, Im SURE I can find something for you to fix while your here, bring your toolbox and all that scanner stuff with you, LOL! :D Mike :burnout
 

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while your in there replkace the wires
have seen several with arcing coil wires












john
 

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Discussion Starter #5
Cardoctor, like I said in the original post, I did wires as one of the first things I've tried, so that's out. Pretty unlikely the new coil wire is giving the same problems as the old one did.

Wyld, I swapped the coil today, granted it was a used coil that I had on the shelf, but it made no change either.

I've got a question that will appear to question yoru judgement here, but bear with me. If a fuel filer will flow enough to let the engine run smoothly to the WOT shift point, why would it not be passing fuel at low revs? That kinda defies my logic. I'm partly hesitant to go after the filter right now as I'm fully expecting to wind lines off instead of getting it loose. I know I'll have to tackle it at some point but I really don't want to right now.

Somehow I think this ratty running is related to the temperature guage not working at all, so tomorrow I'll have a go at getting new senders in there and see it that makes any difference.

Where is Brampton in relation to Toronto? If this thing keeps up you just might see it at your shop!
 

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Brampton is about 20 mins NW of Toronto.

The fuel filter being restricted can cause low fuel pressure.When the miss occurs like that at low speed,the mixture is already quite lean.A bad fuel pump or plugged filter keeps the mixture even leaner,as the fuel pressure is low.The computers know how much fuel it should be delivering based on flow,which is based on pressure and injector duty cycle (on time).When the fuel pressure is low,the computer has no way of knowing,and the system runs way lean,creating a miss.

If you don't want to tackle the filter,then at least check fuel pressure when the problem happens.Tape a gauge to the windshield and go for a drive.You will need an adapter hose to "t" into the fuel line connections at the back of the throttle body.
 

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One other thought is a PROM.Not that yours is bad,but GM releases updated PROM's to correct little bugs that they find.

GM did have a lot of PROM updates for TCC engagement speed,and fuel maps,which caused a lot of similar problems like yours.

A PROM is pretty cheap,and easy to install.It wouldn't hurt to try an updated one before going any further with your diagnosis.

GM dealer is the place to get one.
 

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the 4.3L is a great engine, there are a few pattern failures that they have: is it a vortec (cpi) ? if so here we go.. fuel pressure, fuel pressure, fuel pressure.. big problem! check to see what it is first, then check the EGR valve for carbon, they also have a problem with the fuel lines inside the intake plenuum leaking.
 

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I had similar problem on my 91 blazer it wound up being a bad fuel pump and sending unit.
 

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I was just re-reading the post and thought of one more thing to check- the last time I had problems similar to this on my 4.3 it was actually carbon buildup on the tower of the coil itself. It was not allowing the full voltage down to the cylinders since it was bleed off right at the coil. This allowed it to run fine at an idle, but when it warmed up and I tried to put a load on the motor it woudl massively bog down. I replaced the wires, fuel filter, etc.. but had no luck. I tried a used coil, but had the same problem- until I cleaned the tower after noticing the carbon tracks down the side. After I cleaned it and applied some WD-40 to dry things out it smoothed out and ran fine. Just one more thing to look at if you're still having problems.

Bill
 

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Had same problem

Alan I had the same type problem but very intermittent on my 95 Jimmy, mine turned out to be a bad EGR valve. Pull it off and look for carbon holding it open if not check to see if the valve opens & closes freely. I also noticed a big drop in milage while it was acting up.


Hope it helps, Scott:burnout :burnout
 

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scfall, if you ever have another problem with carbon in your EGR valve again, they make a gasket that has a screen in it to prevent that from hapining.
 

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Discussion Starter #14
Just bringing this thread back to the top. Truck is still hobbling along. Had to change a wye pipe a couple weeks ago and put in a new O2 sensor and did the fuel filter while I was under there. I'm going after it with a vengeance this week, need that truck running right.
 

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Next step is to get that fuel pressure gauge hooked up,and watch it when the problem occurs,at least to rule out fuel supply.

Could you also pull the number off the PROM,and i can check it for any symptom related updates ?
 

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I had a 92 s10 great truck i sold it with 300,00 miles on it and it was running real strong, body had no rust great little truck. Any way It had just those idiot light which i hate. I replaced them with gauges. But stupid me didnt think of the wires i was unpluging would cause anything. It ran fine till cold weather hit and it ran the same as yours i bought it into school me and me teacher opened the hood and the first thing he does is smack me in the back of the head. Then after he made me realize i smaked myself to. hook the sensers back up and she ran just fine. Make sure your sensers are hooked up. oil, temp, volt.

Rich:burnout
 
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