Author Topic: Loss of power  (Read 462 times)

Offline Deathscythe

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Loss of power
« on: August 06, 2018, 09:26:58 PM »
I recently purchased an 86 GMC Sierra Classic with a 305/350 turbo that has an HEI distributor, 4 barrel carb and factory exhaust with a catalytic converter.
When driving at highway speeds, usually around 60/65mph, the power will drop off and it will lose a few mph before suddenly gaining speed to where I have to let off the gas. Humidity and temp don't seem to be a factor, but I was hoping for some knowledge if where to start looking for the issue.
I've had a few suggestions ranging from plugs and wires, replacing the distributor cap and internals to checking the fuel filter or verifying it has a vented gas cap.
Hopefully someone here has had similar issues and a diagnosis handy.
At the moment I'm only getting around
10mpg, which seems a bit low, I'm thinking that might be a side effect to whatever issue there is.
Any help would be greatly appreciated.

Offline Rapid Roy

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Re: Loss of power
« Reply #1 on: August 06, 2018, 09:45:45 PM »
Sounds like possibly catalytic issue to me. But not for sure. You may not know how old it is. IMO.
1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet

Offline Deathscythe

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Re: Loss of power
« Reply #2 on: August 06, 2018, 10:18:59 PM »
The guy I bought it from had new exhaust put on, according to the title he had it for less than a year. He said the shop wouldn't put a non-cat exhaust on since it came in with a cat on it. So, if his information is accurate, the cat is less than two years old.

Offline Irish_Alley

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Re: Loss of power
« Reply #3 on: August 09, 2018, 11:11:20 AM »
10mpg is ok, could be better but its about the norm. from past experience a cat once it gets warm would cut the exhaust flow and limit the output power but it has to cool down before you get that power back. first thought would be fuel issues but i want to wait till BD or vile or someone else chimes in. but just a tad more info from you. is it electric or a manual fuel pump?
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Online bd

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Re: Loss of power
« Reply #4 on: August 09, 2018, 05:01:10 PM »
With the information provided, there are too many possibilities.  I agree with others that you should at least check the spark plugs, fuel filter and the general state of the "tune-up" parts, including fuel supply hoses for deterioration and kinking.  During that process check the fuel pump draw, pressure and delivery volume.  Verify the ignition timing.  Subsequent checks can be performed after you've completed an initial survey.
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In other words... if people learn by making mistakes, by now I should know just about everything!!
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Offline Sheriff_of_Rock_Ridge

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Re: Loss of power
« Reply #5 on: August 10, 2018, 01:13:10 PM »
I had something similar.  I would get to 65 and get no power.  It was shudder a little.  If I held it to wide open throttle it would just keep dying on me.  I would have to let go of the throttle and limp it to the shop.  It felt like a misfire.  The forums I read through had similar problem with fuel filters, fuel pumps, valve on the distributer.  Any how, to make a short story long it was the TV cable.  It was causing it to shift into overdrive to quickly.   I have the Th700r4.  Check the shape of your transmission pan to make sure you have the TH350.  That would have been rare in 1986.  There are plenty of guides online.  Regardless yours might have the electronic shifter.  If it has the linkage it would be good to check the tv cable. 


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Offline Deathscythe

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Re: Loss of power
« Reply #6 on: August 10, 2018, 04:29:07 PM »
Thank you everyone for the information and responses.
I went out yesterday and replaced the plugs, wires, cap and rotor. The cap was the worst, all the contacts were pretty corroded and pitted, I'm surprised it ran to be honest. It has smoothed out a little bit, has the same issue of power dropping off around 65mph and coming back, but bnb it's not as severe. Seems like it happens worse in the morning after a good dew has settled.
As for a mechanical or electric fuel pump, I'm not sure at the moment, I'll be back under the hood this evening. I'm 99% certain it has a turbo 350, not a 700r4 unfortunately. There isn't any linkage cables or TV cables on top of te I take as far as I can tell and the pan is square, no elongated like you'd see in a 700r4.
I'm leaning towards fuel issues now  next up will be checking the fuel filter and finding the pump. The truck has 2 tanks, the left leaks and isn't used, so bnb if the right tank is deteriorating the same, I might find some rust or debris in the filter. The gage doesn't work correctly, either a bad ground or sending unit, I'll be replacing both tanks, sending units, selector valve, switch and lines as soon as I can afford it.

Offline Sheriff_of_Rock_Ridge

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Re: Loss of power
« Reply #7 on: August 10, 2018, 05:25:53 PM »
Mine did the same thing.  Progressively got worse.  Soon I couldn’t get past 45 mph.  Felt like limp mode.  The 700r4 is square with 16 bolts.  The 350 is square’ish with one corner at a 45 degree angle and has 13 bolts.  I’m pretty sure it has a TV cable.  You can look at your carb and see if it’s coming out there.  You should have two cables connected to your throttle.  One for the gas peddle and one for the tv cable.  Either way the throttle and spring would be something to look at too.   


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Offline Deathscythe

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Re: Loss of power
« Reply #8 on: August 14, 2018, 11:57:27 AM »
I got under the hood the other day to work on the wiper fluid hoses and whatnot and took a look around to see if I could spot any issues.
It does have a 350 turbo tranny and a mechanical fuel pump. Timing is good, pump seems to be working fine, it's not leaking and the oil doesn't smell like fuel so I'll assume the pump is fine. I'll be pulling the fuel filter to check if it's still clear in a couple days.
The truck does smell like it's running rich and has a little hesitation right when you get moving. I'm starting to think it may be a bad EGR valve, I'll be pulling that to see if it might be an issue.

Offline Sheriff_of_Rock_Ridge

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Re: Loss of power
« Reply #9 on: August 20, 2018, 06:37:02 AM »
Any update?


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Offline Deathscythe

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Re: Loss of power
« Reply #10 on: August 20, 2018, 09:37:27 AM »
Not yet, waiting for payday to start digging a little more. It has been running better, still a little choppy at idle when I first start it up. Going to look into getting a heat shield for the starter, it seems like it's getting too hot and not wanting to start again after being driven for awhile.
Gotta love old trucks.

Offline haroldwca

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Re: Loss of power
« Reply #11 on: August 28, 2018, 09:03:05 PM »
I don't have a definitive answer, but let me ask you a couple of questions.  In the process of checking things out, did you check the timing?  If you're off by even a little on the retard side, you could be giving up a lot in low end torque under part throttle.

 My second question is a bit of a re-hash, but I will ask anyway.  You are certain that you have the TH350 instead of the 700R4?  I ask because, if you have the standard fare 2.73 rear gears, the 700R4 doesn't hit high gear until 45-50 mph, or sometimes higher.  (I know that it isn't supposed to be higher, but after 32 years and who knows how many people tinkering on it . . . )  That, combined with poorly adjusted timing can give you that "dead in the water" feel as you are pressing the gas further trying to make it come alive again.

I experienced that very circumstance - retarded timing, TV cable out of adjustment, poorly adjusted carburetor - and the result was as you described.  I spent a lot of time looking for the "magic bullet" solution, when in fact it was a matter of re-checking all the basics.

As an aside, my timing was so far off because I had re-used the harmonic balancer from the original 305, instead of acquiring a balancer for a 350.  Until that time, I did not realize that the timing marks on these two items are FAR from matching.  If someone has swapped a 350 for your original 305, they might have made the same mistake I did. 

I hope some of my rambling helps.  Good luck !

Offline Deathscythe

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Re: Loss of power
« Reply #12 on: August 29, 2018, 12:57:16 AM »
That you for your insight and input. It is definitely a 350T, the pan shape, vacuum diaphragm location and lack of a TV cable make it certain. It is running quite a bit better, but there are old vacuum lines and leaks due to cracked or broken fittings. I've accumulated a few parts that I will be swapping out, egr block off, fuel filter, pcv valve and will be getting rid of almost all of the emissions components. I'll basically be left with the main components, simpler vacuum lines and an all around more efficient vacuum system. I have a feeling the degraded nature of most if the smaller components under the hood are to blame for most if the issues I have. The timing was checked and adjusted to around 9 or 10 when I first bought the truck, but the carb probably does need fine tuning, I will be doing that once I get all the vacuum sorted and the emissions components properly taken off.