unior Member
Hi everyone. I apologize for being “that guy” who posts a problem with his second ever post but I am struggling.

I recently picked up an 86 Automatic with some issues. The car will not start without giving it a couple of pumps to the throttle then it will not stay running without a little throttle. The dash is out so I don’t know how many revs it takes to keep it going. I’ve checked fuel pressure and I get 40 psi at key on and it stays there while running. I replaced the MAF after reading some posts and it is a fairly cheap and easy swap.


I had the codes pulled and this is what I found:



Code

Loss of 24x signal



Dtc 36

Loss of e spark signal 2

Mass air flow sensor burnoff circuit 4

Dtc 43 low voltage electric spark control

44 lean exhaust left side

45 rich exhaust left side

46 power steering pressure switch failure

51 faulty prom ecm/ pcm

52 ecm temp low temperature

54 fuel pump mixture or ecm fault

Egr solenoid #2 failure

with all the ECM related codes I decided to swap that out too. Still no change
I noticed that the IAC valve looked all scarred up so I pulled it as well. I replaced it because it wouldn’t adjust at all. After putting it back together I checked for vacuum leaks and I only found one where I forgot to reconnect one hose. I adjusted the TPS to 0.54. Unfortunately I loosened it up before checking so I don’t know what the value was prior to adjusting.
Additionally. There is a lot heat around the center console and it will dump black smoke if revved. I think the heat could be an exhaust leak but I haven’t picked it up high enough to look for that yet

thanks for any and all suggestions

 

Holy smokes ... You're throwing parts at it. 1st clear all the codes by disconnecting the battery for 30 seconds. Start and run it until it comes out of closed loop and then check the codes again. Hopefully some of the codes won't come back. You need a factory service manual and look up the troubleshooting charts for the different codes you get ... check your messages

 

Thanks for the reply. I did pick up a copy of the shop manual. That is almost always the first thing I buy for a new project. I hope to have my OBD1 reader this weekend. I also plan to either borrow or buy a timing light this weekend. I’m not sure if timing will cause my issues but the cover over the distributor is laying in the back of the car. Seems to me a person would only leave that off if they had been messing with it a lot.

 

Just a couple of comments:
First, for starting, don't pump the throttle. On a fuel injected engine, this does nothing. The better way is to turn the key to run, and leave it there for 5 seconds for the fuel pump to build up pressure. Put a slight bit of foot pressure on the gas pedal and start the engine.
Second, check your ignition timing and then re-do the idle speed procedure. There are procedures for both settings on this site, and you should follow them exactly. As 3D said above, get rid of all the codes by disconnecting the battery and start over fresh.
Keep checking for vacuum leaks as well.
Good luck with this one!

 

Did you buy the car this way??
as mentioned before. There is no accelerator pump like a carburetor, so
disconnect the battery, then reconnect and get it to run at least till 150 degrees.
if you can’t leave your hand on the rocker cover, it’s probably hot.
with no gauges hooked up, you’re sort of in the dark.
open loop means the computer is running the engine in cold mode. Its a program in the computer with the only sensors the computer uses being tps and temp sensor.
closed loop means all the sensors are reporting to the computer, and it’s making adjustments based on the info they provide.
most of the sensors are very reliable.
there is no cam or crank sensor, the computer uses the distributor module info using the 4 pin plug behind the intake manifold.
if it needs throttle to stay running, someone messed with the butterfly adjustment, or huge vacuum leak, or egr valve is stuck open slightly.
it looks complex, but everything is a “circuit ” and all the circuits all work independently of each other, leaving the computer to sort it all out.
you just have to figure out if each circuit
is working.
best thing you should do is read the spark plugs. that will tell you if injectors are working properly.
if lean, work towards a vacuum leak, or malfuntioning oxygen sensor.
if rich , be careful, you may get the catalytic converters overheated and burn up.
the center console is where the cat is under the car. If it’s hot enough to make the cup holder hot, it’s rich, and you should, again, check the plugs.
if it’s been that way for a while, on the 86, you can unclamp the cat and remove the cat and mufflers as a unit to see if a melted cat is the problem of why it’s having idle issues.
it happened to my 86. It had the power of a 4 cylinder engine.
one bolt per muffler, one bolt on the torque arm, one bolt on the cat plus air
Tube.

 

Thanks for the info. This weekend I will clear the codes heat it up and read them again. I am also going to check the timing and I will disconnect the cat and check it as well

i did buy it this way. I got it really cheap and was looking for something to work on. Professionally I am a validation specialist which means I look at a computer all day. My Dad is a retired diesel mechanic so I grew up working on things with him and I missed getting my hands dirty. My wife thinks I’m nuts but I am having fun. Even when it is frustrating

 

Just checked the timing and it is spot on at 6 degrees. Haven’t had a chance to disconnect the cat yet

 

After reading other threads on idle issues I decided to repeat the setting process for the IAC. I don’t think I did it 100% correct.

The car will now start and idle without adding throttle. It is a little slow to my ear, no tach, so I think I will repeat the process once more.

 

Recommend that you first A) check and set your Throttle Position Sensor to .540 volts using the FSM procedure, then B) set minimum idle air with the FSM procedure, C) check and reset TPS to make sure it's .540 volt +/- .030, and then D) perform your IAC reset process. If all these things are right settings the car will idle better or at least be heading in the right direction.

 

Update: I am making progress. The car will now start and idle cold. It will even idle in gear. Once it warms up it starts to have issues and won’t start back without throttle or idle. I have learned from all of you very helpful people that means open loop is working and closed loop is where the issues are.

I checked the codes last night and I am now down to 2. Code 43 and code 44. Both the O2 sensor and the knock sensor are fairly cheap and there is a good chance they haven’t been changed in a long time so I am going to pick them up today.

 

Just for your knowledge the Oxygen sensors were designed for a 24 month (or 48k miles) useful lifespan. They are designed to put out a signal when the Zirconium gets to about 600* and this takes two or three minutes of driving or idling. However the C4's with the Un-Heated Oxygen sensors can go into "Open Loop" while idling and the O2 cools down below 600*. If you are at a long light it can do it even.

If you want to know if the catalytic is bad you can pull the O2 and let it run, if a lot of air is escaping through the hole then you might have a plugged or partially plugged Catalytic converter. Usually there is about 5 psi trying to escape when you have a good exhaust system.

On the MAF sensor be very careful as the wires used in them are 1/10th of a mm or a hair. Whenever I replace the MAF I will also replace the two relays that operate the MAF. One is for "Power" to the sensor and the second is called the "Burn Off Relay". After engine shut down the MAF will heat up the wires and burn off any contaminants. This takes seconds and the little wires will glow bright yellow almost white while they are burning off the junk that builds up on a wire as volumes of air pass over it every hour of usage. If either of these two are not working properly then your MAF won't either.

It appears that your EGR Solenoid is bad as well. I would suggest you keep the EGR functional as it is a PIA to remove correctly. The EGR valve uses a Vacuum signal sent by the EGR Vacuum Solenoid to control it. If your EGR is not working then the engine can and might start having detonation or pinging under loads. The worst result of a bad EGR is that the engine will produce Nox which is not good. If you are going to remove it you will have to get someone to fix the code that your ECM uses. You can attach a vacuum pump to the EGR valve and pull a bit of vacuum and the vertical shaft of the EGR should move up and down a little bit and the idle should change slightly.

The Knock sensor is easy to replace and easier to verify it's operation. While the engine is running with the hood open simply tap on your engine with a metal wrench on the cylinder head or anywhere and the idle should drop briefly. I had a bad one that retarded the timing on all 8 cylinders until I replaced it. It made the Corvette feel more like a Chevette power wise.

In the "Closed Loop" mode of operation the engine uses the "Big Three" as I call them. The Oxygen sensor, The MAF sensor and the Coolant Temperature Sensor are the big three that will make the car run in "Closed Loop". It sounds to me like a MAF issue still as that has a lot to do with the drive-ability of the engine. Be sure that there is absolutely NO air leaking into the throttle body from the MAF to the throttle body or the MAF will be made inaccurate and the drive-ability of the Corvette will suffer.
As the oxygen sensor starts to fail the signal will slow down and become narrower and later it will start to lag behind the ECM and this has a bad effect on the Corvette. The new O2 will oscillate between .1 and .9 volts averaging about .450 vdc if you measure it with a meter. Older O2's will start to oscillate between .3 and .7 or whatever and this is not what the ECM wants to see. Keep a fresh O2 on the Corvette fort the best economy and performance! Beside they are cheap and easy to change (normally). Don't use anything on the threads of the O2 other than the factory applied anti-seize. It is very easy to poison a O2 so don't expose it to anything until you are ready to install it, I keep the little plastic cap on them until I screw it into the exhaust

The CTS is very important for the engine to run properly. If this sensor thinks the engine is -40* (F) it will enrich the fuel mixture and make the car run rich, especially if it is 75* (F) outdoors. If the engine mistakenly thinks it is 160* at startup it will do the opposite and reduce the fuel going into the engine. I would suggest that you verify that your CTS is accurate. On my 1988 C4 it is on the front passengers side of the plenum. It will have three wires to it and sends a resistance signal back to the ECM. In the FSM you will find a chart that shows the resistance value and the temperature it represents. It only takes a couple minutes and while you are at it I would clean the connectors of any corrosion as this can affect the resistance value.

You don't want any misfires on the L98 as it will let the un-burned gasoline get into the catalytic and possibly damage it. I replaced my Catalytic with a Higher Flowing unit from Summit and this helps my C4 get better mileage. I have a Cat-back Chambered Exhaust System which will fool the listeners into thinking I have a ferocious engine under the hood, it is loud and sounds great.

Unfortunately even with the ferocious sounding engine I still have the little Toyota Corolla "Race" cars trying to race me every chance they get. It gets embarrassing so I have a new way of dealing with them. I just laugh my *** off when they try and that frustrates them. In Virginia they take away your car with no compensation if you are caught racing on the public streets.

At some point check your battery voltage and then check the power getting to the Fuse panel. They should be the same, if not we can fix that with some electrical connection cleaning. I found corrosion where the 7 fusible links get their power from the battery and a close to 2 volt drop in voltage between my battery and fuse panel. Any Electrical connection should be cleaned as needed. I use DeOxIt which is an amazing product as it wipes away decades of corrosion in one swipe. I use it everywhere on my 2 Corvettes and the family cars.

 

I am making progress but still having issues. I replaced the O2 sensor and the car is driving better but still not idling once warm. It also will die immediately when placed in gear unless I really rev the motor
I cleared all the codes and after a 20 minute drive no codes came back.

If I have read correctly O2,MAF, and coolant temperature sensor are all closed loop. I have replaced the MAF and O2. Will the CTS cause idle issues as well?

 

Yes sir. A bad CTS will cause a lot of issues.

 

Do you have a scanner? Either that or we can keep throwing parts at it till we get lucky or you go broke. Don't bother to set idle by ear. You need a scanner so you can see what is going on and what the speed really is and what the ECM wants to see

 

earlier you mentioned maybe having an exhaust leek. did you confirm this?
i had a bad fluctuating idle issue in my 86 that ran fine when cold but wanted to stall when hot, the car had no codes at all
and after going through setting TPS and replacing the IAC, O2 sensor and going crazy for a while
the problem was an exhaust leek, my driver side "y" pipe was not pulled up to the manifold correctly because the manifold studs were gone and has smaller diameter bolts and nuts
holding it together but not properly, once this was fixed by getting the correct studs installed and no more exhaust leek the car runs perfectly

 

Welcome to the forum. I too have an 86 but its down for a bit with a blown head gasket, I too live in the Indy area... close to the drag strip. There are a few of us on the forum in this area. Good luck with your fix,

 

An update. I have cleared all the codes by working my way through each part. I am still having a surging idle once warm. I read in the FSM that the P/N switch could be a cause. Rather than take apart the interior initially I just put the car in drive and turned the key. It tried to start. After taking apart the interior I have found that the switch is not plugged in. Additionally, I can’t even find the wiring harness for this switch. I don’t see the diagram for this in the FSM. Could anyone point me to where this harness connects just to I can see if they cut it out completely or if it is dangling somewhere?

 

Hello neighbor. Avon( indianapolis) here/. The ground for your instr. panel is by the oil filter. Batee dot com is the go to guy for repair of those panels., I would say lets cruise but I have a head gasket problem. Best of luck,

 

I wanted to come back and give an update. The left side header rusted through and rather than try and fix it I decided to remove the headers and go back with a stock manifold setup. After installing the manifolds and new downpipe along with putting the O2 sensor back where it was originally the car idles much better.