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finished rebuild rough idle

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TNTDSM7777

10+ Year Contributor
89
1
Apr 28, 2011
Missoula, Montana
So just finished my rebuild and my idle is really rough. It starts every time and when I drive it runs decently. Boost builds very good as well. After running for a while it wants to backfire on me. I was just wondering if this is symptoms of bad timing or maybe a boost leak. Cam gears are lined up but my dad did the crank positioning, which I am skeptical about. New coils, plugs, wires, and cps.
 
Could be IAC(ISC). What are your mods, may need to adjust ignition timing to get it to run/idle properly. It could also be a boost/vacuum leak. Either way I would HIGHLY sugest a boost leak test!
 
Ya I was thinking about the iac too. I will clean it with some cue tips and some alcohol. Thanks for the link too. I will do some work on it tomorrow. It hasn't ran in 9 months, so who knows what it could be
 
Does it idle like it's got cams?
Does it idle like it has a misfire and that misfire goes away as soon as you give it gas?

The first one is normally a mechanical timing issue.
The 2nd one is usually from improper valve sealing.
 
More like a misfire, but there are new valves in, which the machine shop did. They inspected everything
 
I have sent numerous heads to the machine shop for valve jobs only to have to take them back off because they were still not sealing.

If it is a dead miss at idle pull a plug wire at a time from the valve cover. If you pull a plug wire and get no change from the engine(miss stays the same) then that is the cylinder that is misfiring.

Check the plug in the offending cylinder, if unsure swap it to another hole and see if the misfire follows.
If it still misses in the same cylinder try swapping the plug wire to another cylinder and see if the miss follows.
If it still misses in the same cylinder try moving the injector to another cylinder and see if the misfire follows.
If it still misfires in the same cylinder then you need to do a compression test on all 4 cylinders.
If the compression is the same on all 4 cylinders or close then you will need to do a leakdown test on the offending cylinder.

To do the leakdown test the cheap way, roll the offending cylinders piston to TDC, using the same compression tester you used to check compression and take the schrader valve out the end of the hose. Screw the hose into the offending cylinder and then apply air pressure to the cylinder usually try 80-100 psi.

If the air escapes back out the intake then the intake valves are not sealing.
If the air escapes back out the exhaust valves then the exhaust valves are not sealing.
If the air escapes into the crank case then the rings are not sealing.

After doing all the steps and the leakdown test shows good then you need to start looking at a wiring issue. Injector wiring especially.
 
ok sounds good. I will try all that in the next couple days. I only have about ten miles on the motor as of now. I read somewhere that it usually takes as long as 50 miles for the rings to seat. If so, maybe driving more will help? I am using Break In Secrets--How To Break In New Motorcycle and Car Engines For More Power as my break in procedure. I will start with easy steps first, then work my way up.
 
ok sounds good. I will try all that in the next couple days. I only have about ten miles on the motor as of now. I read somewhere that it usually takes as long as 50 miles for the rings to seat. If so, maybe driving more will help? I am using Break In Secrets--How To Break In New Motorcycle and Car Engines For More Power as my break in procedure. I will start with easy steps first, then work my way up.

That is the break in method that I use as well pretty much.

That method works wonders but the engine still needs to be running properly for that method to work correct.

If you have a misfire in a cylinder and are trying to break in the engine then the A/F Ratio will be off and bad things can happen.

At the same time I have seen slight misfires that will clear up after just one short 2nd gear pull and moderate boost.


Generally though if the motor was built correctly and the finish hone was done properly then you will not get any misfires from the get go.
 
Ya I understand what you are saying. I didn't take any short cuts during the rebuild, but who knows maybe something could have mistakenly gone wrong. I will do those tests and see what the results are. I might do some pulls too, might as well.

Just got done cleaning the IAC, it wasnt the cleanest, however, it did help idle a little bit. I checked for any misses and none found. Did a couple of pulls in 2nd and didnt help any. Starting to drive it didnt backfire as much. But after 5 minutes or so it just got worse and it was a struggle to get above 2k. Bogging a lot and backfiring a lot. I will continue to check everything else.

Still thinking that the crank wasnt at TDC when timed. I think my dad gave the crank a 1/4 turn back from TDC.
 
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