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Surges under load but only while going uphill

 
cs350
I love my Chevy Chevette!
Posts: 4
Joined: 03/11
Posted: 03/06/11 01:48 PM

Trying to help out a friend.  He has an 86 Chevy C20 with the 350, quadrajet, dual fuel tanks.  The truck starts, runs, and drives fine until you get to a hill. When going up hill it begins to surge, almost stalling but not quite.  It doesn't seem to pop or back fire, just pulls one second, no power the next, and so on and so on.  It's undrivable as it is.  I've verified that both tanks have good fuel in them, the switch over valve is working.  The fuel pump has decent output while cranking.  I rebuilt the carb last year.  The problem surfaced recently after he left the truck sit all winter long.  I disconnected almost all of the emissions vacuum lines to eliminate any of them as a cause.  The vacuum advance is connected.  I disassembled the carb to double check the gaskets, needles, jets, and float.  No problems found.  Anybody seen this problem before?  Anybody got any ideas?  I don't have a spare carb to try so I can't eliminate the quad as the problem yet.  I've never had a fuel pump that couldn't draw fuel from the tank but could push it up to the carb.  I'm running out of ideas as far as what to do/check next.  Any help would be greatly appreciated.  

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idrivejunk
I mow my lawn and find Chevys
Posts: 5119
Joined: 12/09
Posted: 03/06/11 03:14 PM

I have seen curved rubber fuel lines suck shut under a heavy load and act like that. Check ALL pieces of rubber line, especially the one from the fuel line to the pump. Just a thought.  
idrivejunk

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zman123
Everything in my driveway’s a Chevy
Posts: 537
Joined: 06/08
Posted: 03/06/11 04:16 PM

Also check for water in the fuel tank. Sounds like your problem. Heet (fuel additive) will help remove the moisture, then change your filter. I had a similar problem with my truck. water will cause a lot of problems that seem like something else.  

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65SSIMP
Everything in my driveway’s a Chevy
Posts: 549
Joined: 03/09
Posted: 03/06/11 04:51 PM

check the above suggestions first and if that dont work mb check the kick down cable on the trans....might not be shifting down,just a thought. good luck  

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 03/06/11 09:24 PM

change the fuel pump,,, they are cheap. double check all fuel lines.  

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waynep7122
I mow my lawn and find Chevys
Posts: 1130
Joined: 08/09
Posted: 03/06/11 11:10 PM

a 15 second full flow cranking fuel flow test... disabled ignition system...

into a clean plastic 2 or 3 liter plastic bottle...

somebody cranks... while you hold the extended hose into the plastic bottle... watching the pulses of fuel into the bottle..  do they stay the same.. or get smaller and smaller as it pulses...

there is usually a fuel filter inline inside the frame rail..  have you changed that???

which vin number is the 8th digit... L or M   ?????/

is this equipped with an oxygen sensor.???

a heat riser valve????  could the heat riser valve be closing when the vacuum drops??? or a leaking vacuum hose..

since you rebuilt the carb.. was it a feed back carb????  or a conventional quadra jet???

if feed back carb..      do you have a dwell meter that you can hook to the green connector near the carb??? set it to 6 cylinder scale..   and post what the dwell readings are on the mixture solenoid...   idle should be 28 to 35... at idle..

at cruise.. it should be 28 to 40.. or so...   if its stuck above 50 or below 10...   you might have problems..

was this a 305 carb .. swapped onto a 350 swap???    how about an actual carb number from the drivers side???


any kinked hoses????

decades ago.. there was a 440 motor home in the shop i worked at..  my boss at the time was a former MOPAR master mechanic.. and a WWII PBY flight mechanic..  so he was a great tech.. the MH would run great.. go up a long grade .. it would overheat...     larger radiators.. increased cooling fans.. swapped thermostats..  turned out to be a restriction in the fuel line.. where a RV store had installed a new line to the generator set.. and tee'ed into the line from the tank with a Universal tee...  the multi sized version.. you know the one marked NOT FOR FUEL...  they did not cut the small sizes off.. to allow enough flow... so going up the hill.. was just enough to restrict the fuel flow to cause it to lower the float level in the carb causing the engine to build more heat with the lean mixture..  

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cs350
I love my Chevy Chevette!
Posts: 4
Joined: 03/11
Posted: 03/07/11 06:57 AM

Not sure of the vin number.  

It's a carb'd non c3 original 350 engine with original carb.
 
I haven't found any kinked hoses so far.  

I haven't tested the fuel pump output beyond 3 or 4 cranks so I may do that today.  

The truck has headers so there is no heat riser.  

I'm not aware of any fuel filters on the frame rail.  It has the filter in the carb but I haven't seen any besides that.  The one in the carb is still good.  

The problem started after the truck sat unused all winter long.  It's less related to load than it is to pitch.  You can take it out to a level road and floor it from a stop, it'll run fine.  Pulls good, rpms climb till it shifts, nice firm shift, pulls good to next gear.  If you try to go up a hill, forget it.  About 100 ft. into climbing a hill it begins a very severe surge.  Doesn't matter if you are full throttle or part throttle.  The only way to stop it is to let off the gas completely, then you have to try and baby it to get up the hill the rest of the way.  The surge can best be described as a very, very severe egr surge.  The egr on this truck is not connected to an active vacuum hose nor does it have any affect on the engine at an idle when it is opened manually.  

I'm a little bent about this truck because it works so well on level ground under max load but gets stupid going uphill.  I tried to put my old edelbrock carb on it to verify whether or not it's a carb problem but the egr valve is in the way.  I think I'll get a spacer today and try again.

I did try switching fuel tanks which had no effect.  I'm going to assume at this point that the tanks, fuel, and switching valve are all okay.  I didn't see any problems with the fuel lines from the valve to the pump.

Your help is greatly appreciated.  Thank you.  

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 03/07/11 07:38 AM

if you have the quadrajet,,,, that has the "paper element" filter?   remove the filter and pop the little plastic white check valve out.


 my dually 454 truck would starve for gas only when pulling,,,,, went through hell trying to fix it,,,,,, even with a Brand new!!! quad jet carb it still done it,,,,, finally took out the filter (yes ran without it) problem solved!   all it was was the check valve in the filter element.  

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 03/07/11 07:42 AM

OH,,, I forgot to mention,,,, if that fuel switch does not make a complete switch from one tank to the other this could cause a fuel problem..


 simply take a long piece of fuel line and just by pass the switch (yes I know it is a pain to do)   just to make dead sure the tank switch is NOT the trouble.

 the only one that ever gave trouble with me was one on a 87 model with fuel injection,,,,,, both electric pumps in the tanks were running fine but only trickle fuel out of the fuel lines.

changed the tank switch and problem solved.  

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redneckjoe69
I mow my lawn and find Chevys
Posts: 2337
Joined: 03/10
Posted: 03/07/11 08:17 AM

odd problem.  Confused      tranny fluid level o.k.?     ya got me stumped.   runs great when it's on a flat surface?  

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cs350
I love my Chevy Chevette!
Posts: 4
Joined: 03/11
Posted: 03/07/11 06:33 PM

Thanks for all the great ideas.  As soon as I've got something new to share I will post it.  No time today to work on it.  I Expect to make progress tomorrow.  

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cs350
I love my Chevy Chevette!
Posts: 4
Joined: 03/11
Posted: 03/14/11 07:07 AM

It was the fuel pump.  After all the work, after trying 3 different carbs, float levels, timing, plugs, wires, fuel quality, and checking fuel hoses, we decided to just throw a fuel pump at it.  What the hell, nothing to lose, pump's only 20 bucks.  I had no confidence it would work.  Nothing I know would allow for a pump to fail while under light to moderate load going uphill at any speed, but still work fine when under max load on level ground.

Lesson learned: Should have gotten into computers. I nice quiet desk job that pays hourly or salary instead of flat rate.  No really weird funky problems that don't make any sense.  Computers either work or they don't.  No working on a friends truck in the cold rain.

Thank you to everyone who had suggestions and/or input on this issue.  I greatly appreciate the help I find on this forum.  Long live the Chevy v8!!! (And Megasquirt)
Cool  

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 03/14/11 05:59 PM

glad you got it fixed, no problem.Smile  

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skyeking
I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 03/14/11 06:11 PM

Hi there,
  Good luck and thanks for the Megaspiel....Skyeking
   
  Bandits @ 12 O'clock High!!!  
skyeking

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