73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Patman on February 26, 2019, 11:06:20 PM
-
Howdy folks!
Question for you guys:
Ive made great headway on the truck this past week. A buddy came into town and he and i took to wrenching and this thing and we got her on the road.
Now I need to fix the headlights, replace the window motors and lock actuators and rebuild the calipers and do the interior and so many more things, but she’s goin down the road and Im a happy camper. Here’s the thing.
When I go up a decent hill, and I have a few where we live, she really bogs down. I know its not the cat because its only a couple years old. It also passed smog a few days ago here in SoCal so the carb seems to be dialed in fine. Its the original Rochester Quadrajet and its been rebuilt a couple years back. It sat for a year and a half after the rebuild.
So, I just had the trans and transfer Case rebuilt and replaced the cap/rotor and coil as well as the plug wires. WHen I got it back from the trans guy it ran terribly which is how I realized that I had a spark/coil problem. NOw that all of that stuff seems to be managed for the moment, I am having this bogging down under load. It kinda feels like it could be vacuum related but it also feels like its starving for fuel when it boggin down. I want to replace the fuel pump (mechanical) but Im not sure which PSI to use. Summit has so many options...
The truck had a 305 but now its got a 350 with an RV cam and an edelbrock performer intake. Everything else is stock. It stil has the ESC distributor though I did replace the ICM and that resolved a loss of spark after warm up issue I was having a while back.
So, what Im guessing is that my fuel pressure is dropping on an incline and I can resolve that with a new mechanical fuel pump. Just need to know what to look for in a fuel pump.
THanks!
-
ill be more inclined to believe its vacuum related. i broken or bad connection will cause the engine to lean out and bog down
-
Start out with a simple check to see if the secondaries are opening. Have a friend push the accelerator to the floor while you look at the carburetor. Push the spring-loaded secondary air doors open with your finger, and look down into the carburetor so you can see the secondary throttle plates. With the pedal on the floor you should see the secondary throttle plates at 90 degrees and completely open.
What transmission do you have - a 700R4? When it was installed did they adjust the TV cable properly? Or if you have a TH350, is the kickdown cable adjusted?
I'd also verify TDC, plug the vacuum advance line and and get an adjustable timing light to verify the base and mechanical timing. Base should be about 12 degrees BTDC, and mechanical should add about another 20 degrees. You want mechanical advance to come in quickly, preferably all in by about 2200-2500 rpm.
Bruce
-
I would replace the mechanical pump to begin with. Todays fuels are not kind to the diaphragm inside so if its been more than a few years its cheap insurance. I would just match up the pump to whatever it calls for in the parts book to your application. As long as it is a small block pump with the correct number and line size it will work. There is no need to put a "high performance" pump on. The carb can only handle stock pressure with out pushing the needle off the seat and causing a rich condition.
-
The incline will not affect the pump performance.
When you say it 'bogs down', can you describe more detail, i.e. loss of power, dies, have to down shift to keep going...
Is it during the entire way up, or does it bog half way, or at the top?
A lot of variables, need more information for the respondents.
-
This was not a fuel issue. It ended up that the road I was on was a bit rough and the clip that holds the hot wire to the coil on the Distributor Cap wiggled/vibrated loose. Thanks for hte help folks!
-
Well, with these old things it's either fuel or spark, or BOTH!
Glad you found it.
-
I believe that I may have found the main issue all along. After speaking with a carburator guy, he successes that it might be the fuel vapor canister. I have a new one ordered and it;ll be here on Tuesday. Ill let ya know how it goes...
Still having issues after the truck warms up. Ive replaced the carb, distributor, cap & rotor, plugs and wires fuel filter, fuel pump, fuel tank selector valve, sending unit and repaired a few vacuum leaks. Still no resolution.
THe symptoms Im experiencing are:
Once the truck is warmed up, as I’m driving down the road, the motor will sometimes surge a bit, like the engine is running strong, then it’ll all but loose power. It stays running, most of the time but it runs like crap.
I was gonna get my new carb rebuilt by a specialist near LA and he suggested that I check the canister. Hoping that this is the issue. Getting frustrated...
-
cant you unhook the vapor canister to test this?
-
Ok, so this is still an issue. Not just going up hill but rough running in general. When the truck is warming up, I can hold my foot on the gas pedal to keep the truck running and the engine will rev and then it’ll almost cut off if I don’t feather the gas pedal. Sometimes it’ll die and not start up for 10 or so min. Sometimes I can drive it around town for an hour with this rarely happening and sometimes, I cant even get off of my own street. It’s driving me crazy. Sometimes it’ll idle with no problem and drive around with no problem but once it gets warmed up, it starts running rough like someone is either messing with the carb or timing.
I live in california so I need to keep all the smog stuff on the truck to pass smog inspection.
Heres what I’ve done so far.
REplaced the ESC distributor with an HEI and it made no difference. The smog guys had me put the original ESC distributor back on and it actually passed smog.
Rotor, cap, plugs and wires have been replaced
Tanks Selector valve was replaced
Drivers side sending unit (dual tanks) replaced
All vacuum lines replaced and verified for placement
Fuel Evap Canister (charcoal canister) replaced
Ignition control module replaced
Heres what Im thinking.
I can still replace the ESC distributor with a new ESC distributor to see if the issue is in the distributor itself. - Just picked one up at the boneyard that looks to be in great shape. It’s from an 84 but its still the same 4 wire connector so it should be okay...
I can replace the ESC module
I can replace the ECM
I can rebuild the carb
I can see if there is a bad check valve or component in my vacuum system
I can replace the knock sensor
OTher than that, Im at a loss. I will say that my oil pressure pings to 60 as soon as I start it up and that does not seem right to me.
Any ideas or suggestions are appreciated...
-
Is the carb a Quadrajet? If yes, did you check the fuel filter that is in the carb? It gets overlooked quite a bit.
-
Rig up a voltmeter between a verified stable ground and the B+ terminal on the distributor so that you can monitor the distributor supply voltage while you drive. Go for a road test and have an assistant drive. Watch the voltmeter while the symptoms are occurring. Does the voltage become erratic or is it constant at 14+ volts?
-
Yes it is a Q-Jet and I have an inline filter in front of the carb so I dont use the filter in the carb it self. So I have removed the filter and the little spring it sits on.
bd,
Im not sure what the B + terminal is but I have google. I get the idea of what you’re saying though. Thats a stinkin’ great idea man. Thanks!
So I can connect a wire from the B+ terminal on the distributor and run it into the cab, and touch a know ground and observe the voltage. It should read 14+ volts consistently. If it varies too far from that mark when the issue is happening then its something in the distributor, if not then its a fuel or vacuum issue.
Does that sound about right?
-
That's not right!
B+ (i.e., battery positive) is the terminal marked BAT on the driver's side of the distributor cap. It should have a 12-gauge pink wire connected to it. The BAT terminal receives power from the ignition switch.
Connect a voltmeter set to the 20 DC volts scale (black meter probe to a known good ground, red meter probe to the distributor BAT terminal) so that you can clearly see the meter readout from inside the cab while the vehicle is moving. Have an assistant drive the vehicle while you monitor the voltmeter. When symptoms occur, does the voltage remain constant at ~14 volts? (FYI, 14.2 - 14.7 volts is the charging system voltage with the engine running, assuming the battery is fully charged). If the voltage supplying the distributor is erratic or significantly less than the voltage measured across the vehicle battery, then there is a voltage supply problem into the ignition system. You won't know where the problem actually lies without additional tests.
-
Thanks for the correction. I’ll follow up once I get this done. Really appreciate the advice...
-
Ive had a difficult time really understanding what is going with my truck. I think I have the issue understood but not quite sure what the problem is. I still have yet to try the multimeter and positive terminal on the distributor test. Only because the only person that can help me is also my ride if I get stranded. Still below is the full description of the issue im having.
From cold, I can start the truck after a pumping the gas 3 or 4 times, it’ll start but I have to keep my foot on the gas a little to keep it running. About 5min of that and it’ll keep running on its own. Seems to idle a bit low to me. I can let it sit there practically all day long, just like that and it’ll run, no problem. When I drive it, after sometimes 5 min or even as much as 20 min of driving (the hotter it is, the quicker it happens) it’ll feel like a switch was thrown and I’ll lose a near 25% power and the engine runs just a little rougher than before. Then with my foot on the gas and without moving my foot, withn just a second or less, the engine will rev and the truck will take off, I have to let up off the gas just a bit to keep from going too fast. Then it starts to lose power again and will run rough but keep a speed of 45-55mph. However, if I have to drive up an incline, the truck will eventually slow down to 35mph or so and will stay there no matter how much I feather the gas. Sometimes it’ll run so rough that it kills the engine and wont restart for another 5 min or sometimes even 20 or 30 min. After the truck is done resting, it’ll start right up and may drive just fine for the next 30 min or longer even a couple hours once.
While its acting like this, if I have to come to a stop, when I accelerate again the transmission shifts late and hard and I can not find overdrive until I’m at 70mph or more. Down shifts are also loud and hard, kind of klunks into the gear its down shifting into.
But when its running right, it shifts super smooth both up and down and right on time.
The way it happens so instantly, it makes me think it’s a vacuum issue. Problem is, I’m not sure what each check valve on my vacuum system is or what components do what, or even how to test it. Remember, its a California truck so all of that smog stuff has to stay.
I could take one or two pieces at a time to NAPA and just replace everything that way. I cant do it all at once or I’ll forget what goes where...
Driving me nuts. Ive replaced so much, once I get this sorted she’s gonna be a runner...
-
Have you checked your EGR valve? Sounds like it could be getting a vacuum signal when it's not called for.
Pull off the vac line and seal it with a golf tee and drive it.
-
Will do.
-
Still happening...
-
It could be stuck by corrosion of carbon buildup. I would pull it and see if it's gunked up.
Think about it, if it is opening at all during driving, it is allowing exhaust gas to recirculate into the intake manifold, diluting the air/gas mixture randomly.
Do you live where smog is required? if not, plate it off.
-
you should be able to depress the diaphragm by hand and the engine should about stall or stall at idle
-
Ok, I’ll give it a shot.
-
Irish, when I put a hose on the EGR and apply vacuum, the RPMs drop and and it begins to stall.
Does that mean its good?
-
sounds right. i dont even have a clue about how the smog stuff works, but i would almost start where BD suggested
Rig up a voltmeter between a verified stable ground and the B+ terminal on the distributor so that you can monitor the distributor supply voltage while you drive. Go for a road test and have an assistant drive. Watch the voltmeter while the symptoms are occurring. Does the voltage become erratic or is it constant at 14+ volts?
-
Agreed. I’ll try to check it this week...