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96 Blazer 4.3 intermittent poor running
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ejgator
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Jun 16, 2013, 11:35 AM
Post #1 of 14
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96 Blazer 4.3 intermittent poor running
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A lot has been done to this vehicle. Basically, new head gaskets and all seals and gaskets involved. New cam sensor, new crank sensor, salvage yard computer reprogrammed by Chevrolet dealer. Everything was good for a few days.
Now, after sitting overnight, engine runs poorly, check engine light flashes many times. When engine light stays on, code present is P0305 for number 5 misfire has been detected. If the code is then cleared, satisfactory operation is achieved until next overnight down time. Vacuum gauge shows pointer vibration until code is cleared, then shows steady 18 after restart.
Owner now carries code reader with him to get through the day.
Could we be in need of injector cleaner application?
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Hammer Time
Ultimate Carjunky
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Jun 16, 2013, 11:53 AM
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Re: 96 Blazer 4.3 intermittent poor running
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Clearing the code has absolutely no effect on the way the engine runs. It's just storing diagnostic information and changes nothing.
You need to do some diagnostics to determine what is causing the misfire. It could be triggered by fuel, spark or compression issues and that's what needs to be determined through diagnostic testing. Don't clear the code. You are just erasing needed diagnostic data.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
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Jun 16, 2013, 12:23 PM
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Re: 96 Blazer 4.3 intermittent poor running
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I would hope with head gaskets done that valve issues are rules out now.
If you know it's picking on #5 all the time just for quick info swap that plug to another and see if problem follow the plug itself - maybe. That or plug wire?
Basing this on all that work done these things should either be newer or got some trauma in the course of doing things still relying on that head gasket job being right too at the moment,
T
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Discretesignals
Ultimate Carjunky
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Jun 16, 2013, 3:02 PM
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Re: 96 Blazer 4.3 intermittent poor running
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It is kind of odd that when the code is cleared, the symptom goes away. Unless that is a coincidence.
What is the history of why those things were replaced? Did it have the same symptom before the repairs?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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ejgator
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Jun 16, 2013, 3:24 PM
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Re: 96 Blazer 4.3 intermittent poor running
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I know that clearing the code does not correct anything. I just thought the inclusion of this behavior might trigger someone's recollection of a similar situation.
All cylinders have good compression, fuel pressure is 60 PSI at the fuel regulator test port.
Plugs were cleaned and gapped during repairs, the plug wires are new.
When it is running smoothly, obviously spark is good at all cylinders.
Do you have a constructive specific suggestion?
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Hammer Time
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Jun 16, 2013, 3:32 PM
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Re: 96 Blazer 4.3 intermittent poor running
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My suggestion hasn't changed................
You need to find out if the issue is fuel, spark or compression. The only thing you seem to have eliminated is compression, assuming you actually tested it.
The other things you did don't prove anything as yet.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
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Jun 16, 2013, 3:40 PM
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Re: 96 Blazer 4.3 intermittent poor running
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ejgator: If you know now that compression is good move on at HT said. Just because parts are new doesn't mean they are all good. Compression good but misfire on one cylinder points to me a bad plug or wire new or not. You only need to drop a spark plug while installing or anywhere along its travels before they were installed.
Wires by all my own experience most frequently fail to trauma or getting oil soaked. Sometimes just pulling of them instead of properly removing one you can wreck one.
You need to rule out things and on to the next if nothing found.
DS asked for a history which might help too,
T
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Hammer Time
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Jun 16, 2013, 3:44 PM
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Re: 96 Blazer 4.3 intermittent poor running
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You need to determine what is happening before you can determine why it is happening so as I said, you need to determine if you are lacking spark or fuel before you even begin to try to figure out what in those systems is causing it.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
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Jun 16, 2013, 5:42 PM
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Re: 96 Blazer 4.3 intermittent poor running
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You might not have the fancy equipment for checking those things, but for the spark check use one of those adjustable spark testers. The spark should be able to easily jump a 3/4 inch gap which is around 30 KV.
The fuel injector is going to be a little more difficult to check because you have to gain access to the injector circuit at the the connector on top of the intake to check to see if the injector has power and command.
You'll need to remove the cover on the back of the fuel injector assembly connector, so you can back probe the wiring to check for injector pulse. You can rig a NOID lamp into the injector circuit or if you could get your hands on a graphing multimeter, you could see the injector trace. Back probing the connector for testing would eliminate possible terminal spreading.
You can also ohm out the injector coils for each injector using an ohm meter at the pins on top of the injector connector cavity. They should all have the same resistance.
It is also possible that you have an injector command, but you have a clogged up poppet valve nozzle or a mechanical problem with the injector itself. Without a pressure transducer and lab scope on the fuel supply line to see injector pressure drops, it is near impossible to catch a injector flow volume issue with the injector on the vehicle.
A lot of times if the problem is isolated to a clogged poppet, you can either attempt to use an injector cleaner tool, but that usually isn't 100%. Most techs would replace the SCPI with a MFI with the injectors mounted to the end of the tubes. That eliminates the poppet valve problems of the past.
You could also have a cylinder breathing problem even though you have good compression. A running compression check should tell you about cylinder breathing.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Jun 16, 2013, 5:47 PM)
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ejgator
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Jun 17, 2013, 5:18 AM
Post #10 of 14
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Re: 96 Blazer 4.3 intermittent poor running
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I guess you guys have to assume that questions are coming from totally inexperienced people. Let me assure you that is not the case here.
The original problem was coolant in the oil. The owner, hoping to save money, had run straight water for several years. The resulting corrosion ate through the head gasket web. The vehicle, however, was running normally, did not even overheat.
After reassembly, we did a full compression check. I don't remember the exact figures, but I believe we were in the 150 +/- 10 psi range.
This engine has the spyder injection system, so the injectors are completely inaccessible without removing the plenum. The local Chevrolet dealer verified injector pulse when they reprogrammed the computer.
I will apply your suggestions regarding swapping plug #5, also will look carefully at wires and plugs.
DS I'm not familiar with an ADJUSTABLE spark tester. I'll check with my local parts guys.
Does not back probing involve piercing the insulation? I have always been reluctant to do that.
DS, can the injector/poppet assembly be tested if it is removed from the engine? Who typically can do that? I will also look into an injector cleaner tool.
One thing that comes to mind in the area of the injectors.. The gasket set did not include poppet o-rings. If we remove the plenum, we will install new ones.
Replacing the system with a MFI is not an option, the owner is severely financially challenged.
Thank you. gentlemen.
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Hammer Time
Ultimate Carjunky
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Jun 17, 2013, 7:00 AM
Post #11 of 14
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Re: 96 Blazer 4.3 intermittent poor running
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I guess you guys have to assume that questions are coming from totally inexperienced people. Let me assure you that is not the case here.
Yes, we do and so far you haven't proven different.
This engine has the spyder injection system, so the injectors are completely inaccessible without removing the plenum. The local Chevrolet dealer verified injector pulse when they reprogrammed the computer.
Yes, they can be still tested for pulse and fuel volume and the dealer verifying injector pulse randomly doesn't tell you anything about your intermittent miss on #5
Does not back probing involve piercing the insulation? No, it doesn't. It's done specifically to avoid piercing
DS I'm not familiar with an ADJUSTABLE spark tester.
Pretty basic tool in diagnostics
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Jun 17, 2013, 7:01 AM)
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ejgator
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Jun 17, 2013, 8:48 AM
Post #12 of 14
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Re: 96 Blazer 4.3 intermittent poor running
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Once again , HT, you are engaging in useless hyperbole, and not offering any substantive suggestions. If you don't want to help, just try to ignore this thread. OK?
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Tom Greenleaf
Ultimate Carjunky
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Jun 17, 2013, 9:04 AM
Post #13 of 14
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Re: 96 Blazer 4.3 intermittent poor running
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Hey! That was on target and showed the spark tester you didn't understand! That's forcing it to jump vs plain spark and still you can't know if that spark works properly in that isolated cylinder. If the spark test works I blame the plug so far with info provided. Yes a plug can take the current and ground without making a spark for the combustion of still (if accurate) just that cylinder.
Thread is this long already and you haven't rules at least that out. If that check move on to other things that can cause this,
T
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Hammer Time
Ultimate Carjunky
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Jun 17, 2013, 9:48 AM
Post #14 of 14
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Re: 96 Blazer 4.3 intermittent poor running
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No, I told you just what your issues were and what you need to do.
Maybe this is what you really need..........
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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