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gearhead ? on a gm 4.3 vortec
i have a problem on a gm 4.3 vortec. mileage dropped 6-7 mpg in an hour or so. i put in a new maf and O2 sensor aft the converter a few weeks ago. i am getting nominal values on st fuel trims....hovering at + or - 4 pretty much all the time. Long term is another story. -18 to -19 when i am at 1500-2200 rpm. but at about -8 to -10 at idle or below 1500 rpm. this is puzzling why i am getting this rich condition only at speed. all ignition system components are new or working properly. air cleaner new. ait sensor and water jacket sensor are nominal values. timing shows normal. one other thing i have noticed is an abnormal shift on the 2-3 and the 3-4 shift. its as if its not sure which gear it should go to next. i am suspiscious of the tps.....but leaning more towards another failed maf sensor, though that doesnt explain the normal st/lt ft values below 1500 rpm. it hasnt given a rich code yet but it is right on the border of doing so and the tail pipe smells very rich....and of course the 12 mpg screams too rich condition. question...if the aft O2 sensor that was just replaced is malfunctioning, could it cause such a strange set of fuel trims? any other ideas would be appreciated Jun 28 14 08:49 pm Link I'm interested in seeing some of the replies on this thread. I was thinking you might have crap in your EGR valve but when you mentioned that you are also having shifting problems I started thinking that it has to be something tied to the vacuum system since many of these trannies are affected by vacuum. Maybe check the PCV valve? Or look for a vacuum leak. Just a guess from someone who works on his own vehicles but never did it professionally. Jun 29 14 02:12 am Link global vision wrote: How's your engine temps? Jun 29 14 03:02 am Link Not knowing anything at all about that engine (ford small block guy), and assuming that your driving conditions did not change before/after the mpg drop ( ie going from highway driving to gridlock), I would look at your fuel injectors, it's possible one of them has failed/is stuck in an open position. Otherwise, a trip to a mechanic or friend that has a computer that can read the engine codes would be a swell troubleshooting trick. It's a lot cheaper to get a proper diagnosis than to start replacing parts at random. Jun 29 14 03:31 am Link Is your new MAF a GM part or an aftermarket part? I've seen several aftermarket units that were not accurate. A leaky or stuck open fuel injector would cause a rich condition at both idle and 2000 rpm. Unlikely to be fuel pressure problem too. I would suspect a vacuum leak at higher engine speeds. How many miles on engine? perhaps manifold gaskets have a small leak, I see that a lot. Possible fault with evap system but then I would suspect a code would be set. Jun 29 14 07:23 am Link I am not familiar with the quirks of that engine but searching online, other people with the same symptoms usually have: 1. a fuel leak 2. coolant temp sensor problem Based on my findings, and the info you provided; its likely a coolant temp sensor.  The Engine Coolant Temperature (ECT) Sensor information is used by the control module to control: Fuel delivery Torque Converter Clutch (TCC) Engine Spark Timing (EST) Controlled Canister Purge (CCP) Idle Air Control (IAC) Cooling Fan -Paolo Internet Mechanic Extraordinaire Jun 29 14 08:21 am Link I've had a couple of these engines. My current S-10 has about 185K on it. It's a pretty ironclad engine with a coupla - - -um ,, quirks Along with the above suggestions - there are two things I start with 1 - Injectors - they are FUSSY (when brand new and as they get really old) Every 2 years - mine gets a full pressurized cleaning and a follow up tank treatment. I have had SEVERAL times when a host of ills was cured by a good and professional cleaning of the injectors. I would not put it past that motor to have a injector or two that lock open at speed and then return to semi normal operation at idle. They are fussy about everything. 2 - and far uglier - that motor has a history of a bimetallic reaction occurring in the intake manifold gasket - it is usually manifested as a break in the passenger side rear "blind chamber" of the head cooling. The failure leads to coolant in the combustion system in that last cylinder. That - then - deranges the o2 sensor and just about every other sensor related to the exhaust system - Which then makes the whole thing run funky and causes a loss of coolant that is very subtle at first. Replacing the intake manifold gasket is an SOB of a job. (and Yeah, I've done it because my gasket failed at about 140K) Read all about it http://troubleshootmyvehicle.com/gm/4.3 … ld-gaskets Jun 29 14 09:21 am Link I'm not a gearhead by today's standards - engines are way more complicated than those I fooled with "back in the day." I will make one comment about keeping your "innards" clean however, the product mentioned in a previous post - Seafoam - is really good stuff. There is also another product that most wouldn't think about. It is called "Evinrude Engine Tuner." It is just what the name suggests - a product by Evinrude outboards. It does pretty much what Seafoam does, but you don't have to mix it with the gasoline. The product comes in an aerosol can and has a straw, which can be used to squirt the stuff down the throttle body. I had a detonation problem in my 4.3 (Astrovan) and Engine Tuner did a great job of getting rid of it. I used to rebuild old OMC outboard motors as a hobby, so I usually had a can or two around. The engine had about 180,000 miles on it at the time and has over 210,000 on it now. I have had no problems with knocking since. One warning if you are not used to these products - your motor will smoke like crazy for a few minutes after using it. Be prepared for your neighbors thinking you just blew your motor up! Jun 29 14 10:15 am Link I have a GMC Yukon with the 5.7 Vortec V8 that sounds like it's suffering the same problem as yours. I thought a new set of spark plugs, distributor cap, rotor and oxygen sensor would do the trick so that's what I bought. There were a lot of things going on, so I dropped the truck off at my local repair shop (I know those guys) and gave them the full low down. It needed some other work, too (it failed inspection) so I was going to have to buy some parts from them, too. Making a long story short ... when I went back to get it, they told me that the spark plugs in it were original (it's a 1996 model with over 200,000 miles ... I didn't know spark plugs could last so long and do so well ... as were the cap and rotor). They also said they didn't fix the problem. The head technician (this isn't a GM dealership) told me that all Vortec engines will eventually burn the injectors (they're inside the intake manifold) or the valves out. The injectors can't be removed unless the intake is off the vehicle and they can't be tested once they're off so they'll have to diagnose everything else first. If everything else checks out, it's the injectors. They were doing a compression and leakdown test on my truck when I last talked to them (Friday afternoon ... I ran from about 10am until 3am on Saturday into Sunday morning). If the valves are burned out, it's time for me to buy another vehicle. If one or more injector is done, I'll fix it but start looking for another vehicle (they highly recommended large GM SUV's that are 1998 and newer as they have the LS style engines). As the saying goes ... if it ain't one thing, it's your mother. Jun 29 14 08:49 pm Link thanks for all the replies. maf sensor is a delphi unit. made in mexico. go figure. this engine gets the best preventative maintenance possible. all fluids filters etc replaced ahead of schedule..synthetic trans fluid etc. trans is electronic shift. it uses the maf input as well as the vehicle speed sensor and map sensor info to change gears. without maf input it just slams into gear. all temps are nominal. no leakdown on the spider either. and it is a rich condition not a lean one which leaves out any vacuum leaks which would cause a lean condition. also, it is an across the board rich condition...all cyls more or less nominal compression, no loss of coolant or unburned hc in the cooling jacket. i am swapping out the maf from a 5.7 vortec tomorrow which works fine. we will see how that works. its tuned for the larger engine, but it should maintain proper mixture since its going through a restrictive intake......should  i am usually reasonably good at diagnostics, but this one is a bit of a pickle. and as one of you mentioned, its too expensive to just throw parts at it and hope one of them fixes it. not like the old carburetor days. btw....it runs pretty good...as good as a 4.3 could be expected to run anyhow. just sucking gas like crazy  just as a point of interest. i bought a delphi unit from oreilleys about 2 years ago. it lasted 366 days....on a one year warranty.  appreciate any other input from you guys and i will keep ya posted if i figure it out tnx Jun 29 14 10:56 pm Link Red Sky Photography wrote: i have an evap code because i went to an external racing pump and for whatever reason it changed the tank press which trips the low press in tank warning...never have figured that one out....but it wouldnt cause a sudden rich condition since that pump arrangement has been in there 100,000+ mi Jun 29 14 11:01 pm Link If it matters, mine is/was running really rich, too. It would run really well above about 2000 rpm and under load (accelerating) ... if one could deal with the black smoke out the tailpipe. It would surge under idle conditions and be a bit hard to start when warm (it started fine cold). Maybe the MAF is the problem?? I'm going to pay a visit to the shop tomorrow. I will let you know what they say. It may well prove to be the same or a similar issue. Jun 29 14 11:01 pm Link Paolo Diavolo wrote: yeh...i thought of the jacket sensor or the iat sensor, but both are dead on.. i went through a lot of IAT sensors a while back , but this one has been working fine Jun 29 14 11:05 pm Link Lovely Day Media wrote: tnx...let me know what you find out Jun 29 14 11:06 pm Link Sorry about thinking vacuum leak, that's what I see most. Looked at some diagnostic flow charts for rich at high rpm, normal at idle. For that they suggest elec faults with MAF, O2 sensor or ECT. Is it possible you have a flooded charcoal cannister? When it goes into purge at cruise rpm it would be sucking fuel and too much vapor causing a rich condition. Jun 30 14 10:24 am Link Red Sky Photography wrote: i thought about the purge possibility but it would exhaust the pent up hydrocarbones in a few minutes. Jun 30 14 01:25 pm Link global vision wrote: Without being or trying to be anything that carries a negative connotation ... I've never been through that many anything on any vehicle in so short a period of time ... except voltage regulators. On my old Ford truck, it would kill voltage regulators because the alternators weren't putting out enough power and it would overheat the regulators. The other vehicle (not mine) was a Chevy Cavalier or something of the sort. Jun 30 14 02:04 pm Link Lovely Day Media wrote: i thought of that and voltage base readings in the pigtail are nominal at 5 v...as for the map sensors, they just were open circuits on 3 of them and the others were completely out of range. those were bw and also auto zones brand. if it had been a circuit problem then the last one wouldnt have worked. same with the MAF. i had similar problems with an ignition module on a cobra i used to have. went through 5 modules to get a good one. the parts place wouldnt believe that they were all bad. so they started checking what they had on the shelf in stock. all but one checked bad. i have had better luck with delco than after market products, but have had problems with their as well. Jun 30 14 03:58 pm Link global vision wrote: I went through about 8 MAP sensors until someone at the dealer parts counter showed me a service bulletin about an air bleed valve mod kit to keep condensation building up in that vacuum line. It was the condensation that wrecked them. Haven't lost one of them since. Jul 01 14 04:27 pm Link Michael Bots wrote: interesting on the condensate.i dont think that is a problem on this engine, but when i have the doghouse off again i will take a looksee. hadnt really thought about it. Jul 01 14 07:08 pm Link I think the shop is running or attempting to run some kind of scam on me. When I talked to them on Friday, they said they were going to do a compression and leakdown test on my engine to see if the problem could possibly be a valve. They would try having it done by the end of the day (that's fine). I didn't contact them on Saturday or Monday and they were closed on Sunday. When I went back today, my truck still isn't done and they insist on doing a valve job on it. I'm not prepared to spend $1200 to rebuild the top end of an engine that has almost a quarter million on the bottom end, the whole truck isn't in that great condition (it has some rust, holes in the seats, etc etc) and for $1500, I could buy another truck just like it and have a boatload of parts to swap out. They also still haven't done the compression or leakdown test on it. They asked for another day to get it done. I said okay, but it looks like I will not only have to shop for a new vehicle but I'll also have to shop for a new shop and/or tools to start doing this kind of thing myself. All this to say (the point) ... I don't have any information from them on what the problem could be that we haven't already discussed. Jul 01 14 07:51 pm Link I got my truck back today. They didn't fix it. Whether it's because they couldn't or just wouldn't is beyond me but it's not fixed so I'm also shopping for a new shop. They did put the spark plugs and some of the other parts I brought them in for me. The truck runs 100 times better than it did. It starts easier, too. The CEL still flashes, though, and it still shakes a bit at idle. I think they may be right in saying it has a burned valve (maybe more than one) but I can't understand why they wouldn't do the compression/leakdown test they said they'd do. It's not like I expected them to do it free of charge. I'd have to pay whatever they said it cost to do it. Oh well ... if they're not interested in taking my money for doing their job, I'm sure someone is. Jul 03 14 05:40 pm Link Lovely Day Media wrote: yeh...they kinda need to do a compression test before they can see it needs a valve job. i can relate a couple possibities also. first, is the distributor. it is normally made from plastic and wears out very quickly and cause erratic idling and missfiring...and then run nice....and back to crap again. another is carbon on the valves. i used to use pretty much any brand name gas and thought it was all the same. a mechanic friend of mine told me shell seems to burn a lot cleaner and keep the injectors and valves in better shape so i tried it. it took a few tanks of gas but it did seem to help. the more i used it the better it got. it might be worth a try for you Jul 04 14 09:52 pm Link |
