I have a 1987 IROC with 5 speed transmission.

My car will run fine until I'm cruising on the freeway. About 1 minute after I put the car in 5 gear the motor will hesitate and then about 10 seconds later the check engine light comes on. I put in the clutch and the motor idles down and then stalls out. I let the clutch back out while I'm rolling along and it fires back up. The CEL turns off and no problem for another 10 miles but then and it starts the process all over again.

Never sets a code when the check engine light comes on.

Someone told me that they think that the speed sensor might be dying.

Where is it located and how difficult is it to replace?

 

That has nothing to do with the speed sensor. The speed sensor only tells the speedo how fast you are going.

More likely your EGR system is trying to open and is not working properly or your fuel pump is not keeping up with the engine's demand.

If the CEL is coming on, but you can't pull codes after it goes off then it is a temporary code, not a permanent one. This means that the code will only be set while the problem is happening. Again pointing to an EGR or fuel problem, not a speed sensor problem.

 

The ACTUAL FACTS are, your speedo is cable driven, but your VSS is on the back of your instrument cluster. It is a little yellow box wih a ribbon-cable pigtail. It has an optical coupler that literally "looks" into the speedo and "sees" the part that the cable is attached to, whirling around in there. It generates 2000 pulses per mile.

If it turns out you need one, it's QUITE easy to replace. Takes less than 5 minutes to get the dash out and only a few seconds to swap it out. But of course first you have to have another one.

From your description, it's not likely to be either fuel-related, or the "EGR". People like to blame ANYTHING and EVERTHING that goes wrong in these cars, on the "EGR". Ain't so. Just to clear the air, it isn't "timing" either, before someone throws that universal bogeyman under the bus. Let's get all the buzzwords and mysticism and voodoo out of the way right up front, and settle down to some real fact-finding.

Rather than just start replacing stuff because "someone told me", it's a good idea to actually do a bit of troubleshooting.

First, does the speedo itself work? Without it, the VSS has no chance, given how it works. Second, if the light comes on while driving, it sets a code, PERIOD. What code does it set?

 

The speedometer works fine. What happens is very strange. I'll be cruising along at a steady speed of maybe 65 mph on the freeway and out of the blue I'll feel a slight hesitation and then in about 5 - 10 seconds the CEL comes on and the car feels as though it lost power. The light stays on until I put in the clutch. When I do that the tach drops to about 700 rpm and then the engine just shuts off. I let the clutch back out, the light goes off and the engine runs fine again for a little while until the problem repeats itself. There are no codes set in the ecm. It just flashes 12 over and over again.

This just happened to me on Friday on my way to work. When I left work and headed home I took all the back roads. The problem never occured. Today I took the car out for about an hour drive though the park system without one problem. It only seems to happen when I put the in 5th gear and drive at a steady speed.

A local GM dealer told me that the ecm was bad so I replaced it. The problem continued. Then they told me I needed a new prom chip so I replaced it. My fuel pump was bad (only putting out about 25 lbs pressure) so I replaced it and the fuel filter. It runs alot better now with the new pump.

The car only has 52500 miles. it has been stored for several years and is very clean, looks like new. I just wish I could figure out what is wrong.

 

I wasn't just "throwing a buzzword around." When you're cruising on the highway, the EGR opens to do its job. If its damaged or broken, it can cause the air/fuel mix to lean out too far for the computer to compensate. Hence the loss of power and stalling when he pushes the clutch in.

EGR failure and/or and extended lean condition at highway speeds WILL cause the CEL to light up while you're driving and it WILL go back out once the lean condition goes away. It does it every time I take my GMC K1500 with TBI on the highway.

 

When the car first began having problems I did get a code for the EGR valve and also one for the ignition module. I changed the ignition control module in the distributor and it seemed to correct the problem for a short time. Since I didn't know that much about the TPI system I just started listening to the advice from others who had similar sounding problems. I changed the IAC, the TPS, the canister filter, checked for vacuum leaks, changed the coolant temp. sensor, the fan relay, the fuel pump relay, the MAF power relay, the MAF burnoff relay, the ecm, the prom chip, the fuel filter, the fuel pump. Since changing all these parts I no longer get a code for the EGR. It runs pretty good now except for this one aggravating problem.