73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: bob77 on September 07, 2008, 10:58:50 AM
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Ok. At idle, my truck seems to run just a slight bit rough. It's a 30 year old engine, it happens. But, I am barely getting 11 mpg out of that little 250 and I don't get it. I'm looking for a place to start. I have traced all vacuum lines and checked for leaks. So far, I don't see any issues. Am I correct in my understanding that I really only need minimal vacuum lines in order to run? I have made sure that all vacuum lines are as originally run. I NEVER need to pass any emissions where I live, so could someone please let me know what my essentials would be. I am assuming that I could disconnect the lines for the EGR valve and such. Would operating on a minimal Vacuum line system possibly make a significant difference? I really believe that I should be getting at least 15 - 16 mpg out of the truck.
Also, my muffler has blown out. There is a hole next to the outlet pipe that is bigger than the pipe. As soon as I get the time and money to replace it, obviously I am going to. Could this change in back pressure cause any kind of significant fuel mileage issues? I believe that part of the EGR system operates somewhat with backpressure (I think). I am going to have the cylinders pressure tested, but I don't really believe I have an issue there. I changed out the 4.10 rear end to a 3.07 and I thought that would make a significant difference.... I was wrong. I changed out the plugs, wires, cap, and rotor yesterday. None of the plugs were wet or oil covered, but they did have quite a bit of black (carbon) buildup on them.
If anyone has any ideas, let me know. The fella I bought the truck from said he rebuilt the carb, but i had to make a few adjustments to the idle and choke. So, I'm sure that the problem could be in the carb itself. I think I am getting some unburnt fuel coming out of the muffler ( the giant hole in the muffler), but I am hoping to try some other ideas before rebuilding the Monojet. I have never rebuilt a carb before and I am a little hesitant.
Thanks all
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Check for vacuum leaks at the intake manifold to cylinder head.
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It has the integrated intake/head.
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I wouldn't start removing vacuum lines just because it will still run without them. Just check for leaks---idle your engine then take some WD40 and start spraying around all gaskets and vacuum lines. If there is a leak, the rpms will increase instantly.
You can do diagnostics on your engine like compression tests that will help you get a start of where to look. A hole in the muffler won't do anything to your mpg if is after your O2 sensor, if you have one. (being 30 yrs old, i don't think you have an O2 sensor).
Driving habbits will decreae your mpg drastically. Keep your right foot light--no rabbit starts, etc..
Make sure your parking break isn't too tight
Adjust air/fuel mixture correctly on carb
after about 5 minutes of running, your carb butterfly should be completely open
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I do seem to be getting a fuel smell from the exhaust. I thought it was the vent line on the fuel tank filler ( because it was leaking a bit until I replaced it yesterday), but I am starting to think maybe the truck is just running too rich. I can't seem to find any vacuum leak issues ( sprayed all around with carb cleaner), so I think I will try to adjust the mixture.
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Ok. So I turned the Idle mixture screw all the way in, then turned it out about 1 and a half turns. (It had been out about 4-6 turns. Then, I started the truck and turned the idle speed screw in until the truck would hold and acceptable idle.
With the idle mixture screw out so far and the idle speed screw out so far as well, I am assuming that the idle mixture was turned up way too high in order to compensate for the idle speed screw being set too far out to hold an idle. This, (I believe) would have been causing the carburetor to dump much more fuel into the system just to make the truck run. I could be completely wrong on this, as I am still trying to learn a little about carburetors. It makes sense to me, but if I am terribly mistaken, please let me know. Obviously the proof will be at the gas pump, but I am just curiuos as to whether or not I have the principle correct.
Thanks again for everyones help. It is greatly appreciated.
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Have you checked your ignition timing and what happens if you hold your hand cupped over the carb?
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Holding my hand over the carb doesn't seem to do much. It decreases the engine idle just slightly. I doublechecked the timing and it is properly set.
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Ok. Bit of an update. I guess I wasn't getting a ver good seal just putting my hand over the carb. I held a wrag tight down over it and it stalled. A fella where I work tells me that he can tune a chevy 6 by ear... Ok, so I was curious. He fiddled with the distributor for a few second and did find a bit of a sweet spot and the turned the idle mix screw quite a ways out. It does sound a bit better, but I still seem to smell gas when it's running. He tells me that the Idle mixture screw is 3 to 4 full turns out now. That seems wrong to me. It was quite a ways out before and it still seemed to be sucking gas. I suppose I will drive it for a few days and see what happens. If it is no better, I am stumped. I guess I either rebuild or look for a new carb.
One little note. On the top/front left side of the carb there is a Vacuum connection ( I think it's a vacuum connection). It's quite a bit bigger then the other connections and it doesn't seem to appear on my vacuum connection schematic. It comes out of the top, drivers side of the carb and sort of points toward the drivers side front marker light. I'd say it's maybe 3/8 to 7/16 diameter. Right now I just have it plugged. Any ideas?
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If this picture came through, could someone tell me exactly where the Red Vacuum connection in the upper left of the carburetor is supposed to go. I can't find that fitting on my vacuum schematic. Currently, that big fitting is just capped
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Have you looked at this:
http://econtent.autozone.com:24991/znetrgs/repair_guide_content/en_us/images//0900c152/80/07/a3/41//large/0900c1528007a341.gif
http://econtent.autozone.com:24991/znetrgs/repair_guide_content/en_us/images//0900c152/80/07/a3/4f//large/0900c1528007a34f.gif
That red hose isn't used on the Chevy for 1977.
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Well, I am going to replace the fuel pump as I am told it could be leaking a bit back into the oil giving me that gas smell around the engine that I can't seem to locate. I suppose the more research that I do I am learning that 10-11 mpg out of a 250 isn't that far off from what some other folks are getting. I am going to relpace that fuel pump, check the compression, and probably try and rebuild the carb. If the compression checks out ok and the fuel pump and carb rebuild don't help, I will explore other possibilities. Is it true that if I put a 2-barrel on there instead of the monojet and stayed out of the throttle, that I would see better mpg?
Would anyone recommend a particular 2-barrel carb that would bolt right up to my 250, integrated intake/head without too much trouble?
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Well, I am going to replace the fuel pump as I am told it could be leaking a bit back into the oil giving me that gas smell
Only if your oil smells like gas.
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Yeah. I'm not smelling gas at the dipstick. Getting a good deal on a fuel pump and a carb rebuild kit from a neighbor who works at an anuto parts store, so I am going to replace the fuel pump anyway.
I think I am going to tackle the carb rebuild tonight or tomorrow. I've never done one before, but as I understand, the monojets are pretty easy. We'll see...
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BTW, if you want to tune the carb yourself, get a vacuum gauge hook it to the carb and adjust the mixture for the highest vacuum reading.
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Is there a particular spot on the carb that I should connect the gauge?
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Does anyone know what sort of compression reading I should get on my 250. Would this info be on the tag under the hood?
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Vacuum would be connected with an port on Manifold vacuum.
The compression reading won't be on the underhood tag.
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the main thing with compression is they should all read similar. If one reads way off, you have a problem with that one.
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Cool. Thanks a bunch. I have been reading quite a bit on compression testing and am hoping to check it either tonight or tomorrow. (depending on when a buddy of mine brings me his compression tester). As I didn't see any evidence of oil on my plugs when I changed them out, I am more suspicious of a bad valve than of a ring issue. (It's more than 30 years old, I'm sure the rings aren't like new). I am actually more suspicious that something is amiss in the carb. When I do the compression test, I will post my findings and will be anxious to hear opinions.
Thanks as always.
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My '84 GMC has the rochester 2SE 2 barrel carb on the integral head 250. I'm not sure if this would bolt up to your head or not, I'm sure someone here would know.... The 2SE should be easy to find in a junkyard (they came on several different cars as well), and has an electric choke. At the very least rebuild your carb. I went from 13mpg to 18 just rebuilding mine. Cheap and easy, can't hurt.
You can take off your EGR valve and plug the hoses, and fashion a block off plate-- or just plug it and leave it on. No backpressure issues.
You might also consider installing an electrical fan, they are cheap in the junkyards and could give some mpg gain. I put one on mine and felt a difference in engine response. I have done several different things to help gas mileage as a little project, but don't drive the truck enough to check all of them seperately, so I don't know the effects on mpg. My mileage lately is ~16-20mpg ,with a best ever of just over 21.
Jason
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Well, tested for compression tonight. I was VERY pleasantly surprised.
Cyl 1 - 160
Cyl 2 - 160
Cyl 3 - 160
Cyl 4 - 160
Cyl 5 - 165
Cyl 6 - 165
I would call it a give or take 5 or so PSI on these readings as I did each Cylinder about 4 times and kind of tried to take a average. I have to say that I was quite shocked to find that not only did all cylinders compare very closely, but that they were still so high for a 77. I just picked the truck up a few weeks ago. The paint quite worn from the motor, but you can see that it had been painted once. (some parts blue and some red showing through.) I'm guessing that sometime in the last ten years or so the motor had been rebuilt. Other than the leaky valve cover gasket which I replaced, it doesn't seem to lose a drop of oil. The fella I bought the truck from said he picked it up in Nevada about 6 years ago and it had been a Nevada State owned vehicle. (Parks department or something). Some state of nevada tags and such still appear on the dashboard. I'm guessing that the State of Nevada kept up with maintenance on the rig whenever it needed it. I am very pleased.
Now, another development in the Fuel Consumption scenario I am dealing with. I decided to spend the three bucks and replace the fuel filter in the carb. (mildly suspicious that there wasn't even one installed). The gentleman I bought the truck from rebuilt the carb himself a few years ago and didn't exactly do a very good job. Throttle linkage made out of a coat-hanger...etc... Anyway, when I went to pull the old filter, there was no spring behind it and it certainly seemed to just lay very loose in the housing. I figured at the time that maybe that's just how they were....Until...
Tonight, after I installed a new fuel pump, I opened the box for the carb rebuild kit and began looking over what trouble I was going to get myself into this weekend. Low and behold, there on the diagram I see a spring that goes behind the fuel filtr and seats it in place.
Correct me if I am wrong, would missing this spring not allow fuel to bypass the filter and move through the carb at a higher rate than normal? Causing a rich running condition and poor fuel mileage?
I am going to try and locate a replacement spring tomorrow and test my theory (I may be completely wrong... It happens all the time). I still plan on rebuilding the carb this weekend. I'm sure it could use it. If someone would like to give me some feedback on my compression test results and my fuel filter spring theory, I would be most appreciative.
Thank a bunch!!!
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cb7501
Any pointers you could give me on choosing an electric fan or installation would be great. It's something I had thought about but didn't want to pay too much for. I didn't really think about a junkyard item. I suppose just finding a newer chevy truck or some such with an electric fan would do the job.
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I dont know, man. I have always been scared of junkyard parts that are parts that have ever been under any strain. You might install a junkyard electric fan and have it fail in a couple weeks cause it was just used up. If it were me i would get a new one. There must be some company out there that offers an electric fan setup for our trucks?
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Yes there is a drop in for our trucks but it is expensive!
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Yes there is a drop in for our trucks but it is expensive!
you mean this one?
http://www2.partstrain.com/store/?Ntt=Cooling%20Fan&N=9959%201588%204294966976
Yeah thats way to expensive. Or maybe it is the average price of an electric fan. I dont know.
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That isn't a "drop-in". The drop in that uses factory mounting is the Perma-Cool.
http://store.summitracing.com/partdetail.asp?part=PRM-19513&view=32&N=700+4294908216+4294908213
BTW, $260 on the Flex-a-lite is a GOOD price!
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Check again, Capt. That said $360.
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Wonder if the drop in one would still work with that thicker, better radiator?
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Good luck with the carb rebuild, that little monojet will be a piece if cake, even if it's your first carb rebuild! Take pics and notes while you take off the linkage/s and disassemble, it'll save you some head scratching later.
I used a fan from a 98 V6 Mustang, $30 from the local yard, had to modify the plastic shroud a little. Lots of people use a ford taurus fan, do a google search (e-fan taurus) and you'll get lots of info & how to. I bought a fan controller at auto zone (part 733652) for $25, came with the rely and fuse, fixed (pre-set) thermostat. Spent another $8 on wire, connectors, etc. I'm no mechanical genius, and this was an easy project. And for about $60, you can give it a try and see what you think without breaking the bank. You can always 'move up' to a high dollar fan if you want to later -- and have time to put some $ in the bank in the meantime. That's just my 2 cents :). If anyone wants pics of my set up I'll post a few w/ an explanation of what I did.
I've been running mine since this May; mostly trips into town but also hauling. Even in 95 degree weather I had no issues. When I first set it all up I let the engine heat up to ~230 before I hooked the fan up...and watched the temp gauge drop in secods back to usual temp. This thing is impressive. It doesen't even run too often, mostly at stoplights. My stock alternator has been more than adequate, although my truck doesn't have a/c to run or electrical acessories besides a cheap radio...
If you're really worried about the reliability of the e-fan set up, throw your stock fan behind your seat-- you could pull off the e-fan and stick the stock one back on in 10 minutes on the roadside if something goes wrong. Do a little research and pick a good fan, use the correct gauge wire and big enough relay, and you won't be disappointed in this project.
Choosing used vs. new fan:
These fans are well engineered, long running pieces of equipment. The mustang, taurus, and sevaral others have a very good reputation for durability and excellent function even from the bone yard. There's really not too much you could do to 'abuse' an electric fan, so I don't have a problem buying one from the 'yard, as long as all of the wiring & connectors are OK. If it goes bad in 5 years put yer stock fan back on, run to any junkyard and for $30 and you're back in business. For my budget this is the way to go. Lots of people-- from dopes like me in old beaters to guys with highly modified 350's in drag cars--are doing this with good results. I don't have anything against buying a new fan, I just like a thrifty approach to my truck (and an excuse to go the junkyard!). If purchasing new is what gives you peace of mind go for it!
Jason
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Wonder if the drop in one would still work with that thicker, better radiator?
I might be finding out since I my truck in down right now, and it has a much larger radiator in it. I just haven't pulled the trigger yet....
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I would love to see those pics!
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Skunksmash/Bob -- I just posted my electric fan how to over in Members Projects if you're interested
Cruzinhigh has a cool dual e-fan set up from a camaro over there too!
Jason
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The Perma-cool is on its way.