Question about 1995 GMC Jimmy

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Bad idle on my 1995 GMC Jimmy. 4.3 Vortec CPI 4x4. 163k miles. When I turn the truck over, fires right up, and then goes into a sick idle cycle. The idle throttles from 500RPM to about 1500RPM and repeats the cycle. It sometimes stalls at the bottom, but fires right back up. It also eases up quite a bit once the truck is warm. I've replaced: Idle air control valve throttle position sensor Dist cap/plugs/wires/PCV valve/rotor button EGR valve swapped from junk yard fuel filter Several people have told me to replace the injector. I opened the intake and found small puddles of fuel. I do not know how to test the spider. Can I simply turn the key so the fuel gets transported to the injector and look for it leaking? How can I test the spider before investing in the $279 part? How can I test for a Vacuum leak or maybe this is a fuel problem? Oxygen sensors? I'm stumped, and frustrated. Any info/advice is much appreciated! Thanks!

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  • Joshua Arms Jan 20, 2011

    Changed out the entire injector unit, cleaned up the plenim and put a new gasket on. Cleared up a lot of the choking but didn't entirely fix the idle. It doesn't sipke up past 1000RPM anymore, however it still spikes down to about 500RPMs and sometimes dies. I'm assuming someone changed out the thermostat at some point because I believe the stock was 195 degrees. How about the ambient air temp sensor? Exaust leaks or a bad idler pulley? The RPMs do change if I'm sitting idle and I turn the wheel left or right. I'm still hacking away at it!


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Unfortunetly....these spider injectors and fuel pumps can have very similar symptoms when failing. What I do know is I work on this motor more than any other motor in my shop. Very common with fuel problems...and I can comfortably tell you that they updated the injector kits on these car for a reason. The original model is junk. I have done these on just about every one of my cutomers vehicles over 150,000 miles. However, I am not going to tell you that is your problem....but it sounds fuel related. Did/does your vehicle skip? Leaking injectors will usually cause a skip due to shortage of fuel to that cylinder. I also have a computer that allows me to monitor the firing of each cylinder, and do an injector kill test. This is what I would do first. Unfortunetly, sounds like you can't so start with this: Clean your throttle body. The pintle that the IAC motor pushes in and out goes into a small port. Spray that port out with carb clean. Get a fine wire brush and get that throttle body clean and shinny. That pintle can easily stick on existing carbon in that part or not. Check for vacuum leaking with carb clean...spray it around the intake and vacuum hoses...see if any particular spot changes how it runs when you spray it. Do you have a vacuum gauge? what is manifold vacuum? Since you are in the intake...make sure your puddled fuel is not due to pulling the main lines off the the injector assembly. After intake is off. hook fuel lines and harnes back up to injector assembly. key up (no start) vehicle...this pressurizes fuel system. look for leaking. The fuel pressure regulator is built on to the side of that assembly....its the round diaphragm with the tube coming out of it. If it leaks fuel out...that may be your problem. And you can replace that without doing the whole injector kit...fairly cheap. If not fixed....what is your fuel pressure. You need to have 60-64 PSI at key up. Try the carbon build up and intake test first...both quick, easy, and cheap. If this is not your issues...its sounds fuel related....just want to confirm that you don have a misfire/skip. That could change diagnosis quite a bit. Keep me posted....more than happy to help. By the way....I own all GM's't learned my lesson yet

Posted on Nov 14, 2010

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  • Joshua Arms Nov 15, 2010

    I closed the intake back up. I didn't unplug any of the injector tubes at all. I just wanted to have a peek (first time opening an intake and looking at the spider). Now, the throttle body and the intake appear to be one large piece? The entire intake came off in one piece. Is this correct or something I missed? If I spray WD40 around the intake and on the vacuum lines (and I don't know the locations or how many there actually are), and nothing changes, can I rule out a vacuum leak? I'll look for the fuel pressure regulator and check for leaks. Thank goodness I have a Chilton repair manual. I didn't realize how much information is in them. Can I take it into a shop and request those specific tests? I really appreciate your help and your willingness to help. I'll definitely keep you updated. Thanks again!

  • James McIntyre Dec 24, 2010

    The 4.3 has an intake (bottom) and an air intake plenim (upper). The piece you removed sounds like the plenim with throttle body and intake tuning valve in the middle of top. My plenim gasket was broken, hardened over time, and missing pieces. I replaced the gasket and that was the fix. Refer to manual for torque and should be 124 inch pounds and tighten in sequence as stated. I used brake clean and you can use carb clean to spray around intake ports. Engine will try to die with a leak. Vaccuum also goes to power boost unit on Master cylinder and could leak there as well.

  • Joshua Arms Dec 28, 2010

    Thanks for the response. I was definitely the plenim. I did replace the gasket while I was in there. Also put on a new MAP sensor, and throttle position sensor. I have a diagram of the vacuum system from the Chilton's manual I purchased. Its cold out now and I don't know how much I'll be doing over winter (no garage). Also a good opportunity to read more of the manual.

    I noticed something else off as well. The temperature gauge never rises above 200 degrees. Will a temperature sensor have any relation to the idle? I had a mechanic suggest something of that nature but I forgot exactly what he said. Anyways, thanks and any advice is welcome.

  • Susan Faris Dec 11, 2014

    I was having an issue also everyone told me it was the fuel injectors so I cleaned them it ran alright for a month or so then started again I had the fuel pump replaced before I cleaned the spider so I decided that it wasnt the pump so I changed all of the sensors and still had the problem so I bought the 279.00 spider and that didnt fix it I was looking on line and if you have aW in your vin you have the higher pressureso you need a higher pressure pump


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