I've lost my patience today.

My C4 seems to drive how it wants, when it wants. Strangely, when my "Check Engine" is ON, it runs perfectly. And by that I mean good throttle and accleration, good gas mileage, and good idle. Also only likes to run "perfectly" when it's warm outside... about 65+ degrees outside. However, when it's off... I get idle that's all over the place, engine almost dies sometimes at a stop, and my acceleration is terrible. Not only that but my mileage is terrible.

For example:
With "Check" ON... at 50mph in 4th with OD... I'm getting 35-40mpg.
With "Check" OFF... at 50mph in 4th with OD... I'm getting 10-15mpg.

I'm lost, too tired to search for the error, and have too many things on my platter this month to start a scavenger hunt when there's 3 other cars in the stable that need my attention. Not to mention work.

Help me out guys..... I know that I'm ripping out this iron crap in the next year or so and I need to deal with it for now... but 10mpg at 50 is not acceptable.

 

You need to buy the shop manual for the 85 asap.
See link below.

http://www.ebay.com.au/itm/1985-Chev...item4853517f76

Try the base idle setup the ecm wants all this to be right or you will have issues like you are describing.

Do the following when engine is 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 bypass 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.
Of course any vacuum leaks fouled plugs can affect the correct base idle.

The 85 should start easy hot or cold have a smooth idle throw you back into the seat when you touch the throttle and go a long way on a tank.

Good luck

 

If I had to guess I'd say the the maf sensor is acting up, when it is reading, it is giving wrong info to the ecm.

 

My guess is the O2 sensor but you need to verify that by checking the code. It is easy to check the code, once you have the code you can fix the problem and enjoy the car. O2 is probably reading lean and the car is dumping in too much fuel when the check engine light is off.

 

As others have said, get the FSM set ASAP. Its a lost cause without the correct info and diagnostic process to follow. Those 2 books have it ALL--> ! You can get used sets for $75 on fleabay.

Before anything else, read the flash codes. No point in going forward until you know what the ECM knows.

Until the time the books arrive, check fuel pressure. Needs to be 35-40 psi key on. Thsi is NOT your particular problem but you need to look at this to make adjustments for when the car IS right later on.

Now, Go invest in a new MAF power relay. There might be 2 for the 85...power AND burn-off. Buy one and swap them around. If this makes absolutely NO difference, move on...
FSM will have test process for MAF sensor. Relays are the usual suspect.

o2 sensor..? not likely. ALL the o2 sensor will do is lower mpg some...not as much as yours is loosing.

Your p[roblem is almost certainly MAF or TPS related. DO pull the TPS plug and clean the contacts. Adjust to .54v at idle.

Trace all vac lines for air leaks. That causes idle to freak out and float around.

The car runs better with the SES lite because it is then running on a program stored in the ECM ( Calibration -Pak) so it can run when there is sensor failure. When it resets to normal Closed Loop mode its running off corrupt data from a faulty sensor to the ECM so it runs like crap.

Its very probably time for injectors as well. You MAY have one shorted....that will cause it to flood and run very very bad. Pull the dipstick and smell it. IF it smells like gas..DO NOT DRIVE until the problem is corrected. Broken fuel reg or bad inj...flooding the engine with raw gas. Gets in the oil...ruins the eng. Not common but it can happen. This however does NOT go from bad to ok and back & forth...once broken its broken forever.

Until you get it fixed, as a TEMPORARY means of being able to drive it, just unplug the MAF sensor. That forces it to Open Loop operation and it will run off the cal-pak. SES will be on constantly but it will run. This only works IF the problem is electrical or sensor related.

Get the books, buy a $7 relay. That might solve it right there...

 

Just set the idle, but it wouldn't keep running without gas after I set it to about 470. Plugged everything back in and took her for a spin. Ran normally after a mile and idled smoothly at 67-680rpm. Of course, the CHECK light was on during this drive so I'll have to see how it handles the next time it drives without it.

BTW...
Fuel pressure is at about 42psi,
MAF is new,
MAF relay is new (only had 1 to replace),
O2 sensor on left header is new (haven't replaced others if they exist),
TPS sensor is new,
IAC valve is new,
TPS sensor was set to about .54v @ idle and about 4.5v @ WOT,
Injectors are also new and so are the O rings,
Gaskets for throttle body, intake, runners, and manifold are all new.

However, I found that the vacuum line for the fuel pressure regulator was off and to the side. I'm not sure if I forgot to reconnect after the last adjustment, but is it possible that it could have popped out? I usually double check everything I do so it's strange to see this.

 

please use the paperclip and get the ses code so we are not shooting in the dark.

 

from that parts list I'd say that the FSM would definately have saved several hundred dollars in this case...and not even one code to talk about?

this IS an EFI engine... there is an order to how things are done. 1. check/read codes.2 review code descriptions in FSM 3. do base test, fuel press, spark timing.

THEN, order the part (not the whole engine control system, just what is needed) after you have a diagnosis.

OR

you can simply throw parts at the problem and hope to get lucky. But as in this instance, you usually DON'T. and several hundred $$ later it still does not run right.

anybody pay attention to the explanation of whats going on when the SES lite is ON? and why? and why it runs better?

open loop---closed loop----limp home mode ?

maybe not. oh well.

 

I've been reading the codes for months, and most of them lead to changing out parts. For example, I was getting error codes for the MAF which is why I replaced it. The code has given me a 34 which makes no sense because the MAF and relay are new, and the only thing left is an error with the ECM, but if that were the case it probably wouldn't drive at all.... at least that's what I've been led to believe over the years.

And on another sour note, I took the vette out to a friend's tonight and it's still giving me the same symptoms. Seems like I only fixed thew idle when the "check engine" was on.

 

Sometimes an intermittent problem can be a pia to find. I would also check the ecm power and ground connections for any bad connections or corrosion, also the problem circuits. A light tap on the MAF sensor with the handle of a screwdriver will let you know if its good, even if its newer. If the engine stumbles when you tap on it thats your culprit. Access to a scan tool would be a big help also as you can see what the sensors are reading, you may catch the sensor/circuit thats causing the problem and you can narrow your search for bad wiring that way. If the harness has been tapped into using scotch locks/splicers look at these as a culprit even if they look ok.

Hope this helps, Gary

 

The 85 does not use relays for the MAF. It has a burnoff module. The burnoff modules rarely fail.

Checking for codes is an obvious start. But codes don't always tell the story. A scan of the diagnostics terminal during an episode would likely identify the problem. I suggest investing in an ALDL cable ($65) and free software to do some data logging. Then go for a drive and record the data.

 

If you have been chasing codes and they continue to lead to the same things...
then I'd move to the harness. This wire harness is notorious for cross signaling and shorting. The insulation breaks/cracks, signals get corrupted. The grounds on the block are also critical.
I chased a misfiring problem that actually moved from one cyl to another! Finally I found that the inj harness on the driver side was bad, allowing intermittent shorting of an inj...I chopped out THE WHOLE harness, soldered new wire in, with new plugs for everything and it runs perfectly now.
Take your car at idle, and jerk the harness around near the wiper motor. There are a doz cheap splices in that trunk line that DO corrode and come loose...PP signals to the ecm and inj.
Check and clean the harness grounds on the block. 5 wires. hard to find, harder to clean but worth the effort.
Clean the hot wire stack on the jumper pole. I cured a misfire just by wire brushing those connections. Since they supply a regulated amount of power they are sensitive and do not tolerate corrosion well at all.

When you read the flash codes the system error displayed, 34 for example, does NOT mean THAT is the problem...it means that is effected by the problem. 99% of code reading is interpretation of what you see. The ECM codes do not do the work for you. They give you a clue..thats all.
Knowing what I know now, I'd say you are a good candidate for a harness problem.

Thank you for the more detailed info. Thats the point of poking someone....to get a reaction. In this case, the info that I'd like to have in order to help.

mess with the harness and every connection you can find. Ground, power or otherwise.

 

Nobody mentioned the possibility of a bad ECM coolant sensor or connector. That'll make it run rich. But again, without scan data these are all guesses.

 

Codes will tell a lot

 

Is the gas mileage you are seeing (both good and bad) coming from the display, or is this actual mileage? I am wondering out loud which sensor that comes into play for these calculations comes online during closed loop. I used to know this.........

 

This is where live data can be helpful, if a sensor is out of range it may not throw a code. Some codes are only thrown when there is an open or a short. If the ect sensor is out of range, reading cold all the time, it will run rich. There were some years the ect came with a single mushroom type connector which caused false readings to the ecm, I'm not sure if this is one of those years. There was an update to change the ect sensor and connector to the weather pack type of connector.

 

I'm suspecting this, but I don't know if my readings are correct or wrong according to the dash. As far as I've read from other members I'm in the norm, but for $15-$20 it might be worth the replacement.

When it's warmed up, I'll get about 190*F on both oil temp and coolant temp. However, it'll run rich when it's cold and warm.

I'm looking into this because another forum member had my similar symptoms in this thread:
http://forums.corvetteforum.com/c4-t...ct-sensor.html

 

You can't go by what the dash gauge says. It uses a different sensor. The ECM coolant sensor is located on the front of the intake manifold. The gauge sensor is in the drivers side cylinder head. Again, scanning really is necessary. You can buy a cable that connects your diagnostic port to a laptop USB for $65. Then download free software such as WIN ALDL or Tuner Pro RT. You will save time and money by knowing what the sensors are telling the ECM.

 

Funny. Both my wives complain about the same thing?!

 

Well, the problem persists. Still runs and drives normally with "Check" on and gets good mileage, acceleration, idle, etc... but when it's cold outside it really hates it. Drove back from a friend's house after work today and my problems continued. Started with bad idle, it stalled a few times while decelerating from about 40mph, It was running very rich, and also gave me terrible mileage... ex: I give it partial throttle at 50 in 4th gear and I get 8mpg on the dash.

This weekend I cleaned out the throttle body and reinstalled with a new gasket, and also got a new coolant temperature sensor (the one that sends to the ECU, under the TB.)

I need more ideas, about to send this thing to my local mechanic because I'm almost done with this car. Honestly, if it wasn't for the months of work and lack of market for a car that needs work, this thing would be sold.