Igoryan

Hello Corvette owners.
I am new to these forums, sorry if this was already discussed, but i could not find anything similar by searching.
I just bought a Corvette 1986 (earlier model with iron heads).
Seems like it was not maintained well, so i am having a few issues now. When i just bought it, the engine was shaking a bit in the idle and the rpms would float from 600 to 800. I replaced the spark plugs and wires (both were in very bad condition with cracks), the engine became more stable, but it's still a bit shaky and idle floats between 600 and 700. The major issue is MPG. I live near to work and drive just 4 miles per day, and my average MPG is around 10. I only make around 200 miles per tank. And i drive in a calm way - do not push the pedal much and use (D) mode. I can actually see how badly it consumes fuel even when i just warming up the engine.
On the highway the MPG is around 18-22. Due to the official specs the mpg should be around 15-17 in mixed cycle.

Could this be a problem with injectors, oxygen sensor or maybe clogged catalytic converters or is 10 MPG is ok for 1986 car?
And should the 350 engine be very stable in the idle or is that acceptable to have it shaking a bit and the RPMs floating?

Maybe i just making this up and i didn't have any other car to compare to, but I can actually feel that it lacks in performance. I don't feel like it got 235HP, it would not accelerate well, unless you really push the pedal and it makes the mpg go even worse. When i press a pedal, sometimes I can feel a gap before it would start accelerating.

It shows 46000 miles on odometer, but the interior looks a bit used, not sure what is the real mileage.
Apart from that i have oil and coolant leaks from below the manifold - i haven't noticed that at first - will try to replace the gaskets myself.
I've also seen a SES light gone on once, and it went off when i restarted the engine. I am going to buy a cable (maybe here http://www.moates.net/aldu1-and-cabl....html?cPath=64) and try and see if i have any errors in the memory.

383vett

I would complete the tuneup and change the distributor cap and rotor. Then the air and fuel filters and 02 sensor as well. Hopefully this will take care of some of the problems.

Igoryan

Sorry, forgotten to mention - i changed the air filter too.

yd328

If it wasn't maintained well you should also clean out the throttle plates and iac motor. When the engine is cold you should also check to see that the cooling system is full and make sure the engine is getting up to 185 degrees. If it is not getting up to normal operating temp. the computer will not go into closed loop operation and that will effect your mileage.

Igoryan

Thanks. I think i can clean the manifold and throttle plates when i will be replacing the gaskets. Not sure about the injectors.

I think mine is heating up to 205 or around that, then the fan starts.
So it goes into the closed loop operation.

But what about the 10MPGs, is that too much, considering the way i drive and distance i drive?

Docderek05

Air/Fuel, curious if you have checked the condition of your MAF sensor, does it have intact wires? do they have enough crap on them to make cotton candy? are your throttle body butterflys closing all the way or are they crapped up...a few other free checks, or at least a good use of a can of MAF cleaner. and I agree with Frizlefrak on his complete assessment, and also..sounds like a huge vacuum leak somewhere. although I have never done this. (There is a procedure to find vacuum leaks using a hand held propane torch, but I do not have experience with that, so please ask other forum members who may know, for a diagnostic instruction,it's something about propane gas causing the rpms to slightly jump when sucked into a leak, please do not do this...without discussions and getting correct procedure and information about it.)

GKK

I agree on completing the entire tune up including:

Plugs, Wires, Distributor Cap, Rotor, Air Filter, Oil, Oil Filter, Fuel Filter, O2 Sensor, Check MAF Sensor, Set Timing, Adjust, clean or install a New IAC motor and clean IAC passage way (do a search for IAC Motor), Change Transmission Fluid and Filter, Change Coolant and Thermostat, Change Coolant Hoses, Check Tire Air Pressure,

Check Brake Pads and Calipers to make sure calipers aren't dragging on the rotors, Change Brake Fluid, Check all four Wheel Hub Assemblies for wear (jack the tires off the ground and grasp the wheel left to right and make sure there is no movement. Check it again vertically) also, get a can of SeaFoam and let the engine suck it in through the disconnected Brake Booster hose, this will clean all the carbon on top of the valves.

Check the Fuel Pressure and you might also, need to replace the Fuel Injectors if they are clogged or have a bad spray pattern.

This will help your Vette's engine and MPG's a lot.

gerardvg

Check your injectors are all working first you should have at least 30 psi fuel pressure, start the car from cold run for 30 secs and shut her down. Check each header an inch from the head and check the temps are all the same a bad injector will show up here, that is the quickest and easiest way and the injectors are trouble prone. check the Throttle position sensor and do a base idle setup.

It should idle smooth and throw you back in the seat when just tickling the accelerator, they are a torque monster engine and give great mileage even by today's standards.

first thing to do is disconnect and re-connect your battery (or ecm connector near battery) as the ecm will detect faults and make adjustments to try and rectify them. ie retard timing.

Set your car to a base idle (as per my shop manual)
when hot.

Connect a paper clip, to your diagnostic terminal under your dash connect to terminal A and B (the top right two pins).
Turn on ignition but don't run the engine.
wait at least 30 seconds and then remove the idle air valve connector (IAC) (the square plug) then turn of ignition and unplug the paperclip.
Now remove the timing connector a single wire near the fuel pump relay.
Start your vette and set the idle speed to 400 rpm in neutral (auto trans) 450 rpm (manual trans) then check your throttle position sensor,
the top two wires it should be set to 0.54volts dc, also confirm with engine off that the tps rises to 4.5 volts at full throttle.
Turn of the ignition and reconnect the iac valve connector and timing connecter and start your vette.
it should go to 1200 rpm then drop slowly to 600 rpm, take the car for a 15 minute drive with the usual stop lights.
It is normal for a little hunting while the ecm relearns, also do a few spirited take offs.
All should settle after 15 to 30 mins normal driving and have a stable idle. It will get smoother and better with more use.
Of course any vacuum leaks can affect the correct base idle.

Good luck

yd328

I would replace the o2 sensor also as part of a good tune up, I like to start with the basics then go from there. A major tune up, including timing and check your fuel pressure will give you a good base line. Replacing the intake gaskets may also fix a possible vacuum leak, if they are leaking coolant they may also have a vacuum leak.
If you can get the data cable that will be helpful. I like to look at the live data that the ecm is seeing. The coolant temp could be different than what you are seeing on the gauge, it would help check for a lazy or bad o2 sensor and look at the iac counts to help diagnos vacuum leaks,idle problems, etc.

Igoryan

Thanks to everyone! I think i'll start with changing the o2 sensor and cleaning the MAF sensor - that's quite easy and cheap, and check if all cylinders are working, maybe that's a problem with injectors indeed. I will receive a cable soon, so i will try to see if i get any errors from the memory.

mickey5

As long as you're changing manifold gaskets you should consider replacing all the injectors if they don't check out...especially if they are original.

John A. Marker

You can check for codes on you car without getting the cable you are talking about. As noted above, take a paper clip and bend it into a "U" shape. Then take the paper clip and insert it into the diagnostic terminal under the steering wheel above your right knee in the A and B slots. These are the top two slots on the right side. Now turn the key to the ON position (do not start the car). Watch the "check engine" light. The light will blink once then twice (code 12) when you turn the key to the ON position. This code will blink three times. This is just telling you that you are connected and the code are reading. Any stored codes will now blink three times. When all the codes have been show by the blinking light, it will then blink code 12 again three times.

Have a piece of paper handy so you can write down the codes. If NO codes were found, it will keep blinking 12.

Turn off the key, remove the paper clip.

If you want to clear all the stored codes, disconnect the NEGATIVE battery cable for about 20 seconds.

ghoastrider1

I have n 86 as well. You should be able to get sideways from a standing start. 10 mpg isnt right either..these guy have given you good advice. Where you located?

Igoryan

Sorry, I haven't updated my profile - it's Schaumburg, IL.

Thanks for replies. Will try to check everything step by step.
Seems like i was stupid enough to buy a lemon

yd328

It's not a lemon,
It just needs a little TLC. These cars have some years on them.It's just the previous owner hasn't kept up with the work that needed to be done along the way. Most of these cars have been through this already. With all of the help here it'll be running like a top in no time

Gary

ghoastrider1

Correct da mundo. Due to electronics, most folks dont fix these cars. I too, do not believe you have a lemon. My 86 needs all kinds of stuff,but it was cheap and is a ball to drive...worth every penny.Darn good looking vette you have there. Welcome to the forum.

Igoryan

hey guys, just an update.
I got a data cable, but could not make it work with any software except Winaldl. It didn't show any errors, but i could see misfirings and i didn't make a research as to other parameter values. I think i will return the cable.

I ended up leaving car in a shop (didn't have time and proper environment to fix it myself). They smoke tested the intake and found huge leaks in 2 or 3 spots, one of them due to unscrewed bolt that was somehow related to brake master cylinder line - that's why my brake pedal was a bit tight. Computer showed that mixture was too reach, it was adding more fuel due to leaks.
The MAF sensor showed unusually small values even after mechanic fixed the leaks and cleaned MAF, so he tried to install a new one just to see if that would make any difference. New MAF readings were close to normal, so he left it on. I also gave him a new O2 sensor, he replaced that too.
They also cleaned and checked injectors and told me that there were no leaks from manifold gasket or any other leaks - i was misled by other mechanic.

There is still one big vacuum leak remaining through the Throttle Body shaft. I was reading through these forums about TB, i think i will get a TPiS 52mm and send them back mine. One thing I am not sure of is whether i can still run coolant through TPiS TB.

Engine is much more stable now, and runs better. Fuel consumption seems to be better too. It's still a bit shaky, mechanic said it's because back then they were setting up engines to run on lean mixture - not sure about that. I think it would become more stable, when i replace the TB and the the rest of the air will start going through the MAF. They also didn't unplug the battery to clean memory - will do that too.

Otherwise, mechanic told me that it was in very good shape and it looked like the 46000 mileage was real.

GKK

No need for the 52mm Throttle Body. The stock throttle body flows enough air for up to 400hp.

Cleaning the stock Multec Fuel Injectors is not recommended as the cleaning solvents will damage the coil insulation in the injectors as well as the Ethanol in todays fuel which the Multecs were never designed to use. Replace the Multecs with the Bosch III Injectors from member FIC (AKA-John Banner).

Also, bypassing the Coolant through the Throttle Body will keep the Intake Plenum cooler and is a very worthwhile mod.

Igoryan

I would buy a stock one, but they are no longer manufactured. So i have to stick with BBK or TPiS 52mm.

Is it safe to drive like that in winter when outside is 25F or below? I guess i could get problems with valves.

GKK

I've driven my 91 Vette in 25* temps with the TB Coolant bypassed with never a problem.