Just wee bit fustrated

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Croc1990

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So I have a 83 3/4 ton with a 350(from a Chevelle aperantly) sm465 np208 combo.
When I picked up the truck it would drive about 20 min or so and then start to lose power bog down so put it in neutral would go back to normal but once you'd start going again would do the same thing and eventually just die. Couldn't boost it had to let it charge on side of road for 45 min just to move another 15 min and start the process over again.
Guy gave a deader than disco battery so I figured that kiĺed the alternator and that's why it kept doing that.
Put newer battery and alternator in it and set off to go to muffler shop to get the exhaust cause it's been pretty much straight manifolds and low and behold it's happened again
Can someone point me in the general direction I'm just a little flustered and have developed tunnel vision as far as diagnosing goes
 

1987 GMC Jimmy

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I would absolutely have your multimeter with you on this one. I would check the battery before running it and when it gives you trouble. I'd also check alternator output right at the alternator and see what the battery's getting. There may be a problem with that cable. Does the starter drag whenever the battery's fresh?
 

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All cables and connections good and wire running from bat to starter has shorted on me before causing problems
 

Croc1990

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I'll give that a shot tonight I ran to Napa and grabbed plugs and fuel filter and man did it wake that thing up feel kinda dumb not changing it sooner I just got excited to not sound like an airplane

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Croc1990

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As far as a multimeter I have one but seldom use it and I'm getting a little confused about how to use it
Have one similar to this
 

Cuba

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See if u can post a pic of the meter, & we can walk u through it. Show the dial mostly... not too difficult at all
 

Croc1990

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It's a cheap one but I think it will work?

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Cuba

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Yes, that's fine...here's an easy guide using your photo. The basic settings for most vehicle testing: Voltage, ohms (checking resistance), and which ports to attach the leads.
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One of the 1st things to check is the battery voltage, before & after startup. You can use jumper wires to assist in holding the leads to the posts or whatever you're testing.
Before startup should be above 12v... preferably 12.6v. But generally above 12 is fine.
When running alternator should be putting out no less than 13.6v to no more than 15v.... generally a cold start should put out about 14v. This means your alternator is doing its job. We'll go from there.
 

Honky Kong jr

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Use the 10amp scale that's your highest amp setting.
 

Croc1990

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I'm going to try this out tomorrow. Wife and I just had another baby so it's tough to squeeze the time in sometimes
 

chengny

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I'll give that a shot tonight I ran to Napa and grabbed plugs and fuel filter and man did it wake that thing up feel kinda dumb not changing it sooner I just got excited to not sound like an airplane

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If a new in-line filter made such a remarkable improvement, I would stay on the fuel system. The plugs shown above don't really look all that bad - I'll bet it was the filter that helped restore your performance. I am guessing you have a Q-jet.

What you describe below is a textbook symptom of a fouled filter.

When I picked up the truck it would drive about 20 min or so and then start to lose power bog down so put it in neutral would go back to normal but once you'd start going again would do the same thing and eventually just die. Couldn't boost it had to let it charge on side of road for 45 min just to move another 15 min and start the process over again.

One typical example of a restricted fuel filter is a vehicle that cuts out or loses power driving up an incline, such as a bridge. Under load, fuel demand increases and a clogged fuel filter reduces the needed flow. As the fuel filter gets dirty, the fuel pump works much harder. This is why a restricted filter can damage a fuel pump.

A misfire at idle or a vehicle that is hard to start is NOT normally a symptom of a bad fuel filter. Fuel requirements at an idle or startup are low. Even a badly restricted filter can supply adequate fuel under low load conditions. As the need for fuel flow increases, the restricted filter will reveal itself. This is why we notice bad fuel filters on acceleration and when climbing an incline.

The fuel volume test is very handy in determining fuel filter restriction. Pressure will drop when we start the volume flow test. Restricted fuel filters may show adequate pressure with a lack of flow. Reduced fuel volume will cause the fuel pressure to fall under high demand.

Check the carburetor's internal filter - it is small compared to an in-line filter and the media is very fine so it can easily become restrictive to fuel flow.

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Release the supply tubing (be sure to hold the brass adapter/filter cap steady with a 1" wrench to prevent the tubing from getting twisted). When you have the inlet tubing released, just back the brass transition fitting out and check the filter. These trucks didn't come equipped with in-line filters so the internal one could very well be plugged or missing. A missing internal filter might seem like a good way to increase fuel flow, but the lack of one will eventually bite you in the ass. From a man who knows a lot about Q-jets:


Missing Fuel Filter:

Results in: Lack of a fuel filter will allow dirt and debris to get into the carb. This often unseats the needle, resulting in severe carb flooding. I have also seen this cause severe sediment build-up in the float bowl, with eventual plugging of metering orifices, resulting in poor idle and other performance problems such as jammed power piston. Comments: A lot of Q-Jets have had their filters removed. People seem to think that the little filter in the carb must be very restrictive to fuel flow, so they toss it and install an in-line filter. Eventually, somebody removes the in-line filter, but never puts the in-carb filter back in. The in-carb filter is actually very good, and does not produce a flow restriction.

Also make sure that the filter spring is present:

Missing Fuel Filter Spring:

Results in: A missing filter spring will allow dirt to completely bypass the filter. results in same problems as a missing fuel filter. Comments: These springs are commonly lost during filter changes. Many people don’t see a purpose in the spring, so they leave them out. I’ve also heard people say that the spring makes the filter “block off” fuel flow, so they remove it. The spring is essential for proper filter operation.
 

chengny

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I thought the filter in the carb was the only filter

Ah Jeez Dude, I'm sorry - what I thought you were talking about was that you changed an in-line.

But you're totally correct. As stock, the only filter in the system is the one in the carburetor. And even though that internal filter probably does as good - if not better - job than an aftermarket add on, some people insert an additional filter in the short section of rubber hose at the fuel pump:

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So you had already done the right thing - without my incoherent rambling.

But it still might be good idea to trace the supply line - from the pump back to the tank - just to be sure there hasn't been an inline installed by a PO. If you feel performance is back to normal, you don't even need to change it - just so you'll know it's there.
 

Croc1990

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I looked down the line didn't see anything so must just be the one.


I checked things out with the multimeter
Battery was 12.63 volts cold
Wasn't sure how to check alternator so I put the positive lead on the positive terminal on back of alternator and black lead on ground to battery. That ranged from 13.4 volts to 12.9 volts

I got exhaust put on sounds runs way better, guy adjusted my timing slightly was a bit too advanced that's why wouldn't start up after warm.

I haven't driven it much but going to be my dd next week so we'll see

Thanks for the help so far!
 

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Don't forget the intank pick up sock.
 

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