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Problem with 350

 
jasontay
I love my Chevy Chevette!
Posts: 2
Joined: 05/08
Posted: 05/18/08 11:39 AM

I have a 1987 Chevy blazer with 350 TBI.It idles fine but when I give it gas it startes missing .I cleaned the throttle body and it didnt help much . The neighbor come over to look at it and we unhooked the map sensor and its not missing out as bad  .It didnt have a code come up so Im wondering if it is bad or not.It seems to idle high is there anyway to adjust it down some .  

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bryancarz
V-6 Camaros rule!
Posts: 45
Joined: 05/08
Posted: 05/18/08 12:38 PM

How does the truck run under heavy load?   Hmmm...the off idle miss, would you say it is a single cylinder dead miss or a random misfire, or maybe more of a hesitation? If you would describe the symptom as more of a random miss or hesitation I would recomend unplugging the vacum line to the egr valve, if it goes away when you do this the egr valve is overactive. That would mean you probably just need a new valve, if you have restricted exhaust it will cause the egr valve to be overactive. You will usually have a noticable lack of power if the exhaust is restricted enough to cause the egr to be overactive.
 When I diagnose missfires I have acces to an ociliscope designed for primary and secondary ignition system analysis. I understand you don't have access to high end equipment. OTC sells ignition system adapters part #3579 they come in sets of six, you will need two sets. these adapters go inbetween the distributor cap and spark plug wire, you start the truck, get it to misfire then go with a test light or jumper wire connected to ground and short out the spark to each of the individual towers one at a time. when you short one out and it doesn't change engine performance you know that cylinder is not contributing to combustion and you can narrow your inspection to that cylinder specificaly.
 As far as the idle speed is concerned its kinda tough to say without being able to "see" what the computer is trying to do. If you can get data stream, look at IAC counts, if iac counts are at 0 that means the computer is trying to comand the iac motor in as far as it can to lower idle speed. This would indicate there is probably a vacum leak. The minimum air rate could be out of adjustment also but I highly highly highly doubt it, please don't just start turning in that set screw on the throttle body! OTC sells a plug that goes into the side of the throttle body and plugs off all the air that the idle motor uses, then you can set your minimum air rate specs. You must verify base ignition timing and vacum leaks before you check minimum air rate. To check for vacum leaks I would buy a can of brake cleaner and spray along the rails of the intake manifold and where the tbi bolts to the intake, I have seen a ton of those throttle body gaskets leak and cause a high idle. If the gasket is leaking vacum, when you spray at it, there will be a drastic change in engine performance and then you will know you gotta replace it. Make sure you spray around the whole throttle body, its usually easiest to do this with the air cleaner on so you don't get brake cleaner down the throat of the tbi. If the timing is advanced to far the computer can start to loose controll of idle speed. let me know if you don't know how to check that.  
 You could have a vacum leak off one of the vacum lines that comes off the intake manifold. What I like to do is go with a needle nose pliers and pinch off the vacumn lines as close to the intake as possible, if you pinch one off and the idle drops there is probably a leak somewhere on that line. Don't pinch off the vacum line to the map, computer drasticly needs that input.

I hope my input was usefull and makes sense, It can be pretty tough to walk someone through a diagnostic process typing on a computer. Best of luck and keep me posted.  

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jasontay
I love my Chevy Chevette!
Posts: 2
Joined: 05/08
Posted: 05/18/08 06:34 PM

Its a hesitation not really a miss.I'll get some brake cleaner and check for leaks and I'm buying a timing light to check the timing .I've also notice when I put it in gear it lunges real bad .Thanks for the help  

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bryancarz
V-6 Camaros rule!
Posts: 45
Joined: 05/08
Posted: 05/19/08 04:18 PM

if it's more of a hesitation, get it to hesitate and then unplug the vacum line to the egr valve. I've seen a bunch of those egr valves cause the same symptom your describing. I bettcha that's what it is.  

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