Still chasing ghosts on my 87 convert engine re-build. Engine will start cold and run fine until it reaches ops temp. Then, "Check Engine" light comes on an engine wont idle below 1000 RPM. Suspect one of both of the temp sensors on front of intake manifold could be the problem [engine re-build was the result of overheated engine]. But herein is the problem.

Both sensors have two-blade wiring plugs --both are flat pins/blades. The driver side sensors has a traditional wiring connector [coming from the passenger-side engine wire loom]. The passenger side sensor has a connector--much like the fuel injectors. [wire comes from the drivers side loom]. The drivers side sensor fits a 3/4" socket while the passenger appears to have a larger dia thread/plug and fits a 15/16" socket/wrench.

Descriptions vary in my 87 Vette shop manual as to location but manual does make ref to two sensors [three if you count the one-blade connector on the side of the head]. One is a coolant temp sensor and one is referred to as a manifold absolute temp sensor. Again, both are at the front of the intake manifold and both appear to go into the coolant water jacket. There does seem to be some internal coolant leakage on the 3/4" sensor as the wire connector was moist on the inside when I disconnected it.

Ive been looking a Rock Auto for replacement parts but despite having several different temp sensors listed, the pictures appear to be of the same sensor. One of the different sensors listed does require a replacement pigtail/connector so maybe if I can ID the differences in diameter, I can figure out which is which. Any thoughts?

PS--electric radiator fan turns on as soon as I turn key to on position. However, for a time, after initial re-build engine start, fan seems to be tied to engine temp, ie, it didnt start immediately at key on position?????

 

The sensor with the fuel injector style connector is the cold start injector thermal switch. It turns on the cold start injector for 8 seconds (while cranking only). It times out when an internal heater gets up to temperature. It also senses the coolant temperature and will not turn on the cold start injector if the coolant is above 95 degrees F.

The cooling fan is controlled by the ECM. It turns on the fan relay mounted on the wheel well below the brake master cylinder. It determines when to do this based on the coolant temperature sensor output. If this sensor is bad or not connected it will cause a lot of poor running problems because the ECM depends on this sensor for multiple fuel and spark timing adjustments.

You need to pull your error codes and find out why the check engine light is on. I've seen the procedure described in multiple places, but couldn't find it in the C4 tech article section. Basically, short terminals A & B together on the ALDL connector and count light flashes. We need a YouTube video to show how to do this... ;-)

 

Knowing what code was set, will make diagnosing and repair, a lot easier.

Read this http://corvettephotographs.com/c4vettes/codes.htm

 

Once again--great information from the experts on this forum. I was aware of the fault codes but hadnt plowed deep enough into the shop manual to see whether there was a way to display the codes without using a scan tool--now I know! Totally agree with the comments about doing the diagnositcs; otherwise, you are really guessing and some of the sensors are pretty pricey--like the cold start switch/sensor.

So far, there has been no real discernible pattern to the check engine light or the rough idle, except with regard to possible changes in engine/coolant temps. Apparent changes in on/off perameters for the electric radiator fan were also pointing in that direction. Plan remains to take it to a local Vette/hotrod shop for a thorough engine diagnostic but upon discovering moisture inside the wire connector on the coolant temp sensor, it was a fairly cheap fix to see if the aforementioned sensor wasnt at the root of my problems. Should get the new sensor in the next day or so. Again, great advice and counsel from a great forum.

 

The saga continues--replaced the coolant temp sensor this week but rough idle problem continues, though somewhat abated. Appears temp sensor was leaking so needed to be done but again, hasnt solved the rough idle issue. However, thanks to this forum, my education on the deciphering the flashing fault codes has openned up an all new opportunity for self-diagnosis.

Latest "discovery" is a code "34" which addresses most of the symptoms and points to a faulty manifold air pressure sensor--MAPS. Unplugging the sensor appears to solve problem. Challenge now is finding a source for a new MAPS--my current source [Rock Auto] shows the part but it doesnt apear to be the same [ie, MAPS on my 87 appears to be plug-like and screws into the "air box" while the Rock Auto part looks more like a remote-mounted sensor with a hose feed].

Need to confirm difference between these parts but in the interim, was hoping someone has a suggestion on another Vette parts provider--MAPS sensors dont seem to be a commonly available part.

 

Search for a MAF, MAP started in 90.
Diagnosis from my 86 FSM
http://members.shaw.ca/corvette86/Code%2034.pdf

 

Your 87 should have MAF in front of throttle body. End of accordion tube.

 

Not really sure what that Rockauto part is but it's not something from the 87.

This is the MAF (Mass Airflow Sensor) that is installed between the two air intake ducts.


The pic is from the AC Delco website. Their part number is 213-163. This was used only on the 86 and 87 cars. Most large parts stores should be able to find one. Don't bother with the chains like AutoZone or O'Reillys'. Try NAPA or CarQuest.

 

You should call Jason at Vette2Vette.

 

Appreciate the quick input. My bad--just about the time I think I've done enough research to become conversational, what I really know is enough to be dangerous. I am back at square one this morning. So far, I have gotten repeated codes 44 and 46 and on one occasion the 34. Beginning to think I might have an ECM problem but will wait for a second opinion from my local Vette/hot rod shop. They have the tools and experience to cut thru all the false-alarms and "rabbit-holes" I've managed to go down with my inexperience. Again, thanks to all who have tried to help.

 

Here's a link to the error codes for your car:

http://home.earthlink.net/~cliff_har...Misc/misc.html

The ECM in the '86 to '89 range is very unsophisticated when it comes to its ability to detect problems. In many cases it's limited to shorted ("sensor low") or open ("sensor high") connections. One thing to keep in mind is that when you're poking around looking for possible problems (like unplugging the MAF) it's easy to accidentally set an error code that isn't real. The ECM remembers these error codes so they may no longer be valid.

To reset the ECM and clear the codes so you can start fresh, unplug the ECM power connector for a second or so. It's attached to the positive battery cable terminal and may be buried under the brake booster. You DON'T need to disconnect a battery terminal to reset the ECM. Just open this connector momentarily. No tools required.

 

Cliff, that's great advice on the ECM harness disconnect. Ive cleared the codes several times before by disconnecting the bat, which isnt easy, but am sure I may still have gotten a few hold-over codes in the process the last few days. Yesterday was a cool rainy day here in south Texas so I decided on a road trip. I have a jumper wire set up on the ECM diagnostic port so I can easily stop, shut off the engine, ground the port, and decode the flashes.

For the first 20 miles, I was getting intermittent code 34s and the car was running rough. Then, at some point, you could actually feel the problem clear and for the next 60 miles the car ran great, no codes, no rough idle, including several stops. Got back to the barn [my "man-cave"] and shut it down. Re-started it several times and no problems. Let it set for a bit then tried a restart--yup--back to rough idle.

The code 34 points to the MAF sensor but given the intermitten nature, I wonder about the ECM. I do have an extra "chip" for the ECM--dont know its condition but think its the OEM chip replaced by a performance chip now in the ECM. And, I have an extra MAF sensor but have know idea why it was removed or what condition it is in.

As a side bar, if I happen to turn the key to the on position [but not start it], sometimes the electric radiator fan will come on immediately--regardless of engine temp. At that point, I already know the engine is going to run rough. On the other hand, if I happen to turn the key to on and the fan doesnt start immediately--I know the engine is going to start and run fine--no code 34. Finally, when I ground the ECM port to decode the flashes, the ECM doesnt always go to the decode mode right away. Lots of info here but maybe it will give you some ideas. Thanks again.