1987 S10 Runs and Idles good but backfires when gas is given

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  #1  
Old 11-21-06, 02:04 PM
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1987 S10 Runs and Idles good but backfires when gas is given

Hello, I have a 87' S10 2.5L and the engine is rebuilt and it starts and idles just fine but when you give it gas/push on the accelerator, it backfires through the throttle body. Any idea on what's wrong and how to fix it?
 
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  #2  
Old 11-21-06, 02:16 PM
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I'm a planter not a mechanic but that sounds like the timing. Have you checked the timing and/or the timing advance?
 
  #3  
Old 11-21-06, 02:17 PM
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We're thinking it is the timing aswell but not sure on how to check it exactly.
 
  #4  
Old 11-21-06, 02:26 PM
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Do you have a timing light?
 
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Old 11-21-06, 02:28 PM
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Yes I have a timing light
 
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Old 11-21-06, 02:45 PM
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Maybe the people who know about this will correct me but here's what I would do.

Find the engine info sticker under the hood and find out how many degrees BTDC the timing should be set.
Hook the 2 power cords (red and black) on the light to the positive and neg. battery posts.
Hook the clamp on the light to the number one spark plug wire.
Start engine and point light at timing marks on harmonic balancer and see if the proper degrees BTDC are lit up when the light flashes.
Slowly speed up engine and see if timing advances properly.

Be careful and don't get wires, hands, etc. into fan belts, etc.
 
  #7  
Old 11-21-06, 03:02 PM
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Seems to me that a service manual is needed. But check firing order, timing, possibly valve adjustments. I am assuming the engine rebuild was done correctly and that cam timing is correct????
 
  #8  
Old 11-21-06, 04:32 PM
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When using the timing light and letting it idle, the timing mark is normally between 9 and 11 o'clock and when giving gas sometimes does advance a little.
 
  #9  
Old 11-21-06, 05:14 PM
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this is how a farmer would proceed

You need to find out what the timing should be for your particular engine. My old Ford is 8 degrees before TDC at idle, if I remember. Yours may be some degrees before or after TDC. Find the little metal pointer connected to the block and pointing at the harm. balancer. Get somebody to bump over the starter until the timing marks come around to that pointer. Take a piece of chalk or something and mark the number of degrees you're looking for before or after TDC on the balancer. I think each mark on either side of TDC is 2 degrees. If the timing is right the light will light up your mark and the pointer and should also advance as the engine is sped up.
However this will only check the timing. Unless you have a distributor on that Chevy, I couldn't tell you how to set it if it's off. The timing on my Ford isn't adjustable. 87 Chevy may not be either.

This will give you something to do until a mechanic can give you more help. Stay out of that fanbelt.
 
  #10  
Old 11-21-06, 05:46 PM
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You have piqued my curiosity. You say the engine was rebuilt, WHEN?? Just recently and this is the first time running?? Or has the engine been running well previously?? The reason I ask is that if this is the first time running after an engine rebuild; then the possibility of a valve timing sprocket mis-alignment exists. One tooth out of whack would let the engine idle normally, but backfire at high rpm as an intake valve may be slightly open when it should be closed. Keep us posted.
 
  #11  
Old 11-27-06, 03:58 AM
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I have forgotten to post an important point on this subject. About the engine being rebuilt, only the block was rebuilt. The head wasn't rebuilt. The block was rebuilt after I blew it up racing a car on the interstate and dropped down from 5th to 4th gear. Do you think that the problem could lie within the head and if so what would be the best way to go about it? Rebuild it or buy new one etc. I also have another engine with everything on it. This engine was blown up aswell by different circumstances.
 
  #12  
Old 11-27-06, 12:16 PM
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Make sure you don't have a couple of the plug wires crossed. Some engines will exhibit the behavior you describe when this mistake is made.
 
  #13  
Old 12-02-06, 10:28 AM
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People around here have been suggesting that the timing is still 180 degrees off. I personally have no idea what that means, how to determine if it is and how to fix that.
 
  #14  
Old 12-02-06, 06:20 PM
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to set the base timing you have to ground the aldl connector by jumpering pins a and b just like you was getting codes from the computer, then you adjust the timing on a warmed up engine by turning the distributer and adjust it to specs it should tell you this on the under hood sticker and tell you what timing should be set at, believe it may be 8 degrees btdc dont really remember.
its not 180 off wouldnt worry about that but you might check the cap and rotor to see if it has any moisture inside it as it could cause backfiring also.
 
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Old 12-02-06, 06:25 PM
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This truck doesn't have a computer.
 
  #16  
Old 12-02-06, 06:35 PM
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an 87 s10 with a 2.5 throttle body fuel injection engine does not have a computer wonder how the injector works without a computer to control it? the aldl connector should be under the drivers side dash.
 
  #17  
Old 12-02-06, 08:00 PM
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ok smart arse lol.....it does have the computer stuff but if you hook up the code reader thing, it says there's an error and to check your connections and stuff. I do and all seems well. Also I don't have an underhood sticker.
 
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Old 12-02-06, 08:04 PM
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OK..............Lots of possibles here So I'll try to go through them as best I can..........I'd first check the firing order, inspect the dist cap and rotor for cracks/wear..........wires and plugs..........basic stuff.

To check the timing, you're gonna have to read the data sticker under the hood. Either there's a brown wire you're gonna have to disconnect or you're gonna have to put a paper clip in the ALDL (diagnostic plug) to bring the engine to a point where you CAN do a base timing check/adjustment.

If memory serves me right, that spec is gonna ask for an AVERAGE of cyls 1 & 4.............so put your timing light on the COIL wire.........
I HAD an 89 and had a similar problem with mine...........turned out to be a bad ignition coil..........That's just a throw in.........

One last tidbit of info........On the 2.5 when the cyl head is overhauled........If the head is shaved, the machinest also needs to triim an equal amount off each valve stem as there is NO ADJUSTMENT for the valves......only a torque spec for the rocker studs
 
  #19  
Old 12-02-06, 09:20 PM
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wouldnt worry about what a code reader says if you jumper the aldl and the check engine light starts flashing you should be able to set timing.
 
  #20  
Old 12-03-06, 01:07 AM
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If this is the 6 cyl, I've had this on a Blazer- it was 180 degrees out of time. Disconnect the battery. Take off the valve cover and watch the valves while you hand cycle the engine. The intake valve for number one will open, then close and the piston will come up (you can feel it through the spark plug hole). At this point the timing mark should be at, or close to, TDC. Put the mark right on TDC and take off the distributor cap. Ensure that the rotor is pointing at #1 spark plug. If it is *not*, you will either need to take off the timing chain and move the camshaft 180 degrees, or rotate the distributor 180 degrees. If the distributor cap is in the right position, take off the timing chain cover. In the manuals, they have you set the timing chain with the marks aligning but TDC is with both marks *up* (or visa versa). Ensure that the timing marks on the camshaft and crankshaft align correctly (you'll need a manual to be sure).
 
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