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03-21-2011, 12:09 AM | #1 |
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Join Date: Mar 2011
Location: Mission Viejo, CA
Posts: 1
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Drivability specialists..I need help!
Thanks for clicking on the thread. This is my first post on this discussion board. I've been here a couple of times for advice on my 1990 3/4 ton Sub 7.4L TBI. Truck is all stock except for a performance chip.
This has been an ongoing issue since I bought the truck about 2 years ago. When I start the truck cold, it runs very rough and backfires through the exhaust. The longer it sits, the worse it gets (one week is worse than overnight). I've also noticed that it has really poor mid throttle performance even after is warms up. The idle is a little high, but anything from 10% to about 90% throttle feels like it's running lean. Here is the list of parts I've put on over the last 2 years: Complete intake reseal MAP sensor TPS O2 sensor CTS TWC Spark plugs, cap, rotor, wires ECM (swapped CAL pac chip from old) Added a performance chip Fuel pump and filters I've checked, double checked, and triple checked the ignition timing and fuel pressure. The injector spray looks good. Compression is good warm (haven't checked it cold). Old changed regularly and motor is very clean on the inside. I've had a lot of experience on OBD II cars, but not so much on OBD I cars. I do have access to a scan tool and the integrator is 133 last I checked (running lean, commanding rich). I've disconnected the EGR valve and it still feels like it's running lean (a trick I learned from when I worked at Saturn). I haven't done anything with the throttle body. My next thoughts where to replace the pressure regulator and injectors possibly Any ideas? I remember my '92 with pretty much the same drivetrain had a similar issue without the backfiring cold start. I installed the complete Edlebrock multi point conversion on it with the same issue (feels like it's running lean) Oh yeah, one more thing, it has always passed CA emissions testing. Hope I can get some help. Thanks again for any possible help. I look forward to sharing my knowledge with the board whenever possible |
03-21-2011, 12:50 AM | #2 |
yes, i do
Join Date: Jun 2007
Location: Spokane, wa
Posts: 2,734
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Re: Drivability specialists..I need help!
check to see if you have a vacuum leak. the throttle shafts on the tbi unit tend to wear out and will cause a vacuum leak
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ERROR 404....... SNAPPY COMEBACK NOT FOUND _______________________________________________________________________________________________ -60 Chevy C10, dead 350/good th350 combo, sitting on a 76 c10 blazer frame, built from 11 different vehicles "the abomination" -07 gmt900 silverado 1500 lt xcab z71, granite blue -81 suzuki gs650g "shelah"- current project -81 kz1000m1 csr "sarge" -next project (just beautification) -07 kawasaki vulcan classic vn900 |
03-21-2011, 06:46 AM | #3 |
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Join Date: Feb 2007
Location: Memphis MI
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Re: Drivability specialists..I need help!
X2. Very common on high mile TBI. A reseal of the TBI won't fix it.
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1987 2 ton 1982 250/TH350 beater in progress Dad's 1981 3/4 L6 3 on tree posi and no options, awaiting restoration or scrapping Plus a mess o' tractors |
03-22-2011, 03:00 PM | #4 |
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Join Date: Aug 2009
Location: St. Paul MN.
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Re: Drivability specialists..I need help!
did you spray carb cleaner around the throttle body gasket to check for a vacuum leak
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Fuzzy |
03-22-2011, 03:06 PM | #5 |
Has more rust than truck...
Join Date: Apr 2005
Location: Ivanhoe, MN
Posts: 2,421
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Re: Drivability specialists..I need help!
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03-22-2011, 06:30 PM | #6 |
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Join Date: Sep 2010
Location: hingham ma
Posts: 1,721
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Re: Drivability specialists..I need help!
longer it sits,, the tbi shaft wouldn't change..
but a sticking valve would,, dump a qt of marvals in the oil.. it show lean if the charge is going right out the exhaust valve, and when fire'n you get a backfire.. if you're lucky you didn't burn the valve yet |
03-23-2011, 04:46 PM | #7 |
It's Better With Nitro
Join Date: Apr 2008
Location: Chino Hills, CA
Posts: 2,262
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Re: Drivability specialists..I need help!
...one other thing to check...check to see if your exhaust manifolds are cracked. I had this issue on a '93 and it cause all sorts of problems similar to what you described.
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1963 C-10: Deluxe-optioned cab, shortbed, fleetside Pontiac 462 ci, Kauffman D-Port alum. heads 4L80E, narrowed sheetmetal Ford 9-inch Tubular front and rear suspension Custom 6-piston front disc and 4-piston rear disc brakes |
03-24-2011, 03:48 PM | #8 |
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Join Date: Oct 2003
Location: OC CA
Posts: 1,374
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Re: Drivability specialists..I need help!
Back firing through TB indicates that ignition timing is off or you have a valve opening at a wrong time. On high millage engines OE timing chain is known to stretch causing valve timing to be inconsistent. During 70's and early 80's GM used to install plastic cam/crank gears in a stupid attempt keep engine noise down. I have seen timing chain jump a tooth or two resulting in wrong camshaft timing vs. piston travel! Since you have replaced every sensor on your TB it looks like it might be engine mechanical and not EFI related issue.
Check your harmonic balancer outer diameter ring - high millage balancers are known to slip exterior ring (due to deteriorating rubber) resulting in incorrect ignition timing indication. Run a compression (or better cylinder leak down test) check - if there is a stuck (or burnt) valve that will catch that easily before doing anything else. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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454, backfire, drivablility, tbi |
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