ok so my 6spd 89 has random hiccups ( instantaneous check engine light) lasts a second or less, car bucks for the second check engine light go's on. does not have any codes stored. ive tried to monitor volt gauge while this happens but never saw any changes. never happens at idle, has never happened as far as i know in any gear but 5th or 6th meaning i think its engine load related. of course ive checked battery connections etc... when cruising it probably happens every 2 or 3 minutes. have never seen it happen because of a big bump in the road, today for the first time car died, i was going fast enough that i left car in gear until i pulled over and stopped. car started immediately and for the next 20 min or so never hiccuped again, since car is a manual and i left it in gear car should have started again on its own, but never did until i cranked it over. i have no egr ( was deleted in tune) also have headers and a mini ram. car runs awesome this started maybe 3 weeks ago. also doesnt seem to happen until coolant over 180deg. again also seems engine load related because only happens in 5th or 6th when slightly lugging motor

sorry so long. any ideas. im a good tech and dont want to randomely start swapping parts

 

I have a similar thing going on with my 91 6 speed.

Low rpms in 5/6 lugging the motor a little. It’ll hesitate and maybe misfire occasionally. If I down shift it takes off without issue.

Im going to look under the cap today. I’m thinking spark...I’ll check base timing too.

Fuel pressure check can’t hurt either.

Car is tuned and problem has slowly came about. Sometimes it’s fine. Sometimes it happens.

edit...I’ve had TPS sensor issues in the past. Poor low rpm performance. Never threw a CEL...just flashed once or twice. If you replace yours, make sure you go with an AC Delco only.

 

You need to hook up a scanner and see if your MAF sensor or the Oxygen sensor is doing something abnormal.

I have found that cars under loads that have a bad MAF or even a bad MAP in the car can and will hiccup and miss while under loads. I recently changed one in my Buick where it was missing while towing a trailer on hills. I replaced the sensor and the problem was gone.

When you have an occasional hiccup and a flashing SES light at the same time for me it has been oxygen sensors "starting" to fail. I have had that same experience with two different cars that had failing O2 sensors.

A failing O2 sensor can be detected by somebody with the right tools but that comes with a cost usually. If you have a decent multi-meter you might be able to do it, You look for the oscillations between .1 and .9 Vdc. Anything over two years or 50,000 miles is done for and needs to be replaced, they are very accurate when new. To me it is just another wearing item that you replace every so often. This does not mean I replace every O2 sensor every two years as today they are better and last longer.
Older O2 sensors don't react quickly enough and this has a negative affect on the ECM. Older MAF/MAP sensors also have age related bad effects. Both of these two items I have mentioned are primary inputs while the car is in closed loop operation. The loss of either would cause a problem for the engine. Older O2 sensors keep the car "trying" to get a smooth idle and can result in hunting rpm wise. A defective MAF sensor would send wrong information regarding the volume of air being ingested into the engine per fraction of a second. The result would be a Air Fuel ratio incorrect for the vehicle under a load. Either or both sensors might have to be replaced but I am pretty sure you are on the way!

Having a scanner would allow you to see the issue. It really sounds like one of the main components as I mentioned above. If you have any problems let us know, somebody here will know how to help you out!

Best regards,
Chris

 

I swapped out the cap and rotor and it feels better. Old one had carbon tracing on it.

I have a new o2 sensor (3 wire)...I’ll try that.

Map is original to the car....28 years old. Maybe it’s time to chuck one in and pray. What brand would you roll with?

my 1991 wrangler used the same MAP. Maybe it’s time to try a swap. Such a shame putting mopar on a Chevy.

 

I have had issues with carbon tracking before but usually with smaller distributor caps and aftermarket ignition Coils. The aftermarket coils can make the air ionize and that in turn made the carbon tracks in my smaller distributor cap. I removed the Hi performance coil and the problem went away.

Did your 1991 C4 come with a MAP sensor on it? My 1988 C4 has a MAF sensor on it and I have replaced it once since I have owned this C4.

After reading a book about the C4's fuel injection systems I learned to look for the primary components that fail and cause issues. When the C4 is cruising down the highway in "Closed Loop" mode the computer is relying on a couple of critical inputs. Air volume is one and the O2 sensor is the second and the CTS is important at this phase to keep the mixture where it needs to be for economical operation.

One thing I did learn is that having a "heated" O2 sensor would be more reliable and last longer as it goes farther back in the exhaust stream.

Good Luck with your Corvettes, I hope that this helps with your missing while under loads!

Best regards,
Chris

 

thanks, i was thinking o2 sensor myself.. it is 3 years old. it is a heated acdelco. i upgraded to heated when i did all the motor work.
I have an old mt2500 snapon scanner that should be able to scan and i can datalog with my laptop also. but working two jobs very little time

 

I think I got mine squared away.

1991 had MAP from factory.

Old cap and rotor has some gray carbon tracing where the spark jumps from the rotor to the cap. Tried cleaning them with sandpaper. Made it worse. Replaced cap and rotor and it ran better but not right.

Swapped In a new AC Delco heated o2 sensor and disconnected the computer to reset all the fuel trims, etc. car ran considerably better as it adjusted / learned.

Bought a BWD Map sensor from advance. All ac ones I found were reboxed China junk. BWD states made in USA.

Car seems back to being 100 percent. As it relearns I’ll see if it degrades back to where it was. There was a flat spot from 1200-2000 rpms. It’ll pull there now and throttle response is much better. Saving the OEM map just in case.

like mentioned above...once in closed loop the cpu / tune takes inputs from the MAP / MAF, CTS and o2 sensor. That’s all it has to adjust. I wish the MAP and o2 was easier to test. I could have hooked up the new laptop but it was easier to throw 50 bucks and two hours of my time at it and see how it went.

Where can you see the value for the CTS without a scanner? I know the one on the head does the analog gauge, and I think the one in the intake does the ecu value and you can see that one in the ac programmer under value 16. I cannot see a digital value from the one in the head like you can in later C4s.

Thanks for your help.

 

The CTS is located under the throttle body and has three wires going to it. It is easy to test as you get the resistance value and then see what the temperature being displayed is. In the FSM there is a chart that has the various values and the corresponding temperature.

MAP sensors are a pain in the butt but better than the MAF sensors they replaced. The MAF has the small wire that keeps getting dirty and burning off over and over. The MAP sensor is a smaller more compact sensor that provided the same information to the ECM.

Carbon Tracking is DEATH to a distributor cap, I have never been able to recover the parts after carbon tracking occurred.

The one on the cylinder head between cylinders #1 and #3 is part of the B4P auxiliary cooling fan that is a pusher fan mounted in front of the radiator. I am not sure on the 1991 but that is what is on my 1988 C4. My B4P sensor is just a switch that goes to ground to turn on the auxiliary cooling fan, it does not have any corresponding temperature. If you ground the wire that goes to the sensor your aux fan should start up.

Heated O2 are so superior to their earlier counterparts!

It sounds like you are "On the road" again with your Corvette! I hope that it works like new for you!

Best regards,
Chris

 

This might help some with sensor values. I got this off one of the Corvette sites.

C4 Corvette Sensor Information

C4 Corvette Sensor

Sensor Values and Location

C4 Corvette Sensor Coolant Temperature

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[img]file:///C:\Users\cdm74\AppData\Local\Temp\msohtm lclip1\01\clip_image001.gif[/img]

• 185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F. Front of engine, below Throttle Body
• 1985-91 Located Under Plenum
• 1992-96 Located In Water Pump

Engine Oil Temperature Sensor

• 185 Ohms @ 210 F, 3400 Ohms @ 68 F, 7,500 Ohms @39 F. Left rear of engine, just above the oil filter

Oil Pressure Sender/Switch

• 1 Ohms @ 0 PSI, 43 Ohms @ 30 PSI, 86 Ohms @ 60 PSI. Top, left hand rear of engine

Fuel Quantity Sender C4 Corvette Sensor

• 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full. Top of fuel tank, beneath filler pipe escutcheon (shield) panel.

MAT (Manifold Absolute Temperature Sensor)

• 185 Ohms @ 210 F, 3400 Ohms @ 70 F, 15,000 Ohms @ 40 F. Underside of manifold air plenum at rear.

Outside Temperature Sensor

• 4400 Ohms @ 60 F, 2200 Ohms @ 85 F. Right side of engine, top right corner of radiator

In Car Temp Temperature Sensor

• 4400 Ohms @ 60 F, 2200 Ohms @ 85 F. Coupe: above left seat near interior courtesy light, Convertible: center of cargo compartment lid.

MAF (Mass Air Flow) Sensor

• .4 Volts @ idle, 5 Volts @ Full Throttle. Front of engine ahead of throttle body

Oxygen (O2) Sensor

• .1 Volt Lean Mixture, .9 Volt Rich Mixture. Left side of exhaust pipe near engine block

TPS (Throttle Position Sensor)

• .54 Volts Idle, ~ 5 Volts Full Throttle. Right side of throttle body at the front

 

Well I still have the miss.

Going to reset the ecm and see see if it goes away and relearns the moss back in

going to check fuel pressure next

going to try to get my laptop hooked up and watch and see what it’s doing.

Time will tell.

 

Car finally threw a CEL, didnt want to run, etc.

h41 and h74 in the ccm

ECM was replaced with a Delco reman in May 2015

plastic ecm mount is cracked. It was missing one screw closest to the master cylinder. I think it shook itself to death again.

So I’ll try to sturdy up the mount, throw it in, and see how it goes.

Will report back in a few days

 

Did you ever figure it out?

new ECM helped me but not the answer.

fuel pressure was normal and not dropping off.

finally double checked base timing it was set to 4*

changed it to six and all is well

start with the easy stuff first!

 

i have a 1988 c4 coupe 4+3 . the car regularly hiccups in 2nd gear when departing from a parked spot when hot, sets service engine soon light but no code. other then that it runs perfect. i have replaced fuel filter, fuel pump,oxyg sensor ,ign module ,cap ,rotor ,wires , airfilter , oil pressure sender and reset the ecm . any ideas?

 

In the '88, if you are seeing the Service Engine Soon light but there are no codes.... something is up.

Are you sure you're pulling the codes correctly? After the SES light flashes 1,2....1,2....1,2... you just see 1,2...1,2...,1,2 again?