divotdug

I've been searching and reading a lot of threads regarding my issue.

Thought it best to start my own just to explain my issues as best I can. I'm still trying to sort out what is normal for this engine and what is not.

I'm sure the fine folks here will point me in the right direction.

So I recently bough this '84, 4+3 car. Pretty well documented at 35k miles based on MVA records. Looks like the the last dozen years the car has mostly sat just making an every other year visit to the emissions station, which it has always passed.

The car starts just fine at around 1200 rpm when cold, working it's way down to 800 rpm within 2-3 minutes. Idling perfectly smooth. Then for about a minute or so the idle drops down to a very rough 700 rpm. Within about a minute the idle smooths out.

I have put about 500 miles on it since I bought and it runs quite nicely...until tonight. Wife and I were going to a local car cruise about 15 miles away.

Had no problems on the way out. Stopped by to pickup dinner. Took about 15 minutes. Came out, car started but would not idle below 1500 rpm. Thinking I might have developed a vacuum leak we started home. Got a 1/2 mile the car stalled and would not start. It would fire up but immediately died as though it was out of fuel.

Some good Samaritans helped push us off the road. While waiting for the tow truck, the car started though still idling high.

We made it the 15 or so miles home but with the car continuing to idle high UNTIL we were about a mile from home, idle went back to normal and the car started running just fine.

So now I'm thinking coolant sensor and maybe gummed IAC valves. Sound like I am going in the right direction?

The coolant sensor appears to be original. It still connected with the big piece of white plastic. I have a new one so no big deal to replace.

But I do have a question, just how the heck does the old one come out? Does the big hunk of plastic just slide out with the connector attached?

Thanks in advance for everyones time!!!

GQ-ROD

If your coolant sendor is original, replace it with the later model one, also replace the pigtail connector, solder and heat shrink the wires.

To remove the old plastic connector you pinch it then pull to release it.

As for your idle, check for codes, you may have a bad map sensor OR just a brittle vacuum hose TO the map sensor.

Check your timing, check your ignition, check your fuel pressure. Report back as you check and find results.

Your idle speed is too high, should be around 600 when hot in gear.

https://www.corvetteforum.com/forums...or-to-ecm.html

ex-x-fire

I'd consider removing the top intake plate so you can reseal it & the tbi base gaskets. Usually you find other leaks too.

Bendeco

High idles will be the coolant temp sensor at the front of the intake manifold. It is broken and not reading any temperature and the car thinks it is really cold out so it runs high thinking it better or the engine will quit in these cold temperatures that the coolant temp sensor isn't reading at all.

Hot Rod Roy

What ex-x-fire is saying is there may be a vacuum leak somewhere that is causing your problem. I'd agree with that, and the top plate of the intake manifold is a common problem area. But before removing it, try tightening all of the bolts in the top plate, including the two weird mounting bolts for each TBI. Removing the TBI's from the top plate should be a "last ditch" effort, because that will foul up the balance between the two TBI units, which is a considerable amount of work to re-balance. My '84 has 240,000 miles on it, and the TBI's have never been off the top plate, nor re-balanced, and my car runs fine.
If you remove the top plate, it can be sealed with RTV Gasket Maker. You shouldn't need to replace the gasket. Check all of your vacuum hoses for wear or cracks.
Each TBI unit should have a cap on the vacuum port that is used for the balancing operation.

divotdug

Thanks to all for giving me a pretty good checklist to work from.

I know small blocks and carbs but I'm not going to pretend to be any sort of EFI expert.

I recognize the car is 32 years old. Replacing parts and wiring for reliability is on my to do list. Just need guidance on most apt parts to replace first. I already picked up the coolant sensor. If I even remotely suspect the map sensor I will replace that as well.

I also plan to pretty much go through and replace all the vacuum hoses. Hopefully it doesn't have 5 miles of hoses like the '72 I had 40+ years ago.

Just a couple more questions.

Is 600 rpm the same idle point for both the auto and manual transmissions? 600 just seems low but if that is what it is supposed to be, not problem. I also have a 4 speed '76 Camaro with a mildly warmed over 350. I have that idling at 750.

On pinching the plastic cover on the coolant sensor, where is the pinch point? Closer to the block or the fat part away from the block?

Thanks all!!

WVZR-1

Pinch points are easily seen in this post. Searching eBay for either part number will get you a visual of the connector. REPLACE IT WITH LATER PRODUCT - it's certainly the more desirable for reliability!!! Information for later more desirable is in that thread also!

https://www.corvetteforum.com/forums...post1592189688

Hot Rod Roy

There's been confusion on "coolant temperature sensors". There are three of them. On the '84's (ONLY), the one on the right (pass.) side, between plugs 6 & 8 is for the radiator cooling fan relay. The one on the left (drivers) side between plugs 1 & 3 is for the temperature gauge. The one on the front of the intake manifold, below the thermostat, is for the ECU. My ECU temp sensor does NOT have a pinch style connector, and I believe it's original. It has the "snap finger" style two pin connector. Only my radiator cooling fan relay connector is the "pinch" style. The "pinch" style is removed by squeezing the "fatter" sides of the connector, then pull straight off. It took me a while to figure that out!

I'm sure WVZR-1 knows the answer to this question.

There is also a "pinch" style connector on the knock sensor.

divotdug

The sensors for the gauge and fans seem to be working just fine. The temp gauge works just fine and the fans kick on as expected near 230º.

Definitely talking about the ECU temp sensor. Hope to get it replaced tomorrow.

FASTAZU

You may also want to check the hose in the tank that connects the pump to the fuel line, If its never been changed its a good bet it has swelled and the plastic clamp broken = no fuel pressure. Also here is a great post on the CF. Its from the C3 section as the 82 also had CF.

https://www.corvetteforum.com/forums...solutions.html

divotdug

A quick update.

Checked all vacuum hoses and connections and everything seems in order.

Checked intake top plate and found no lose bolts.

Replaced the ECM coolant sensor with the new style. That smoothed the idle right out. Only got to drive a few miles because of impending storms but can't believe what a difference that sensor made.

I'm going to go ahead and drop in a new fuel pump and strainer later this week.

Thanks for everyone's advice!!

divotdug

Update #2.

Car is running very nicely, no more rough idle (knock on wood). Only time and more driving will tell.

Still not idling at the suggested 600 rpm. Consistent 700 rpm. Quite possible my 32 year old tach is a bit off.

Pulled the IAC valves. Was surprised to find one of them only finger tight. They both looked pretty loaded up with carbon. Instead of cleaning them, I went ahead and replaced them.

Also decided to drop in a new fuel pump and strainer. The strainer was pretty dirty but I don't think enough to inhibit fuel flow.

I opted to use the 1985 pump in lieu of the 1984 pump.

I used a Delphi pump.

FYI, the pump's electrical connection on the Delphi pump was not the same as the 1984 pump. Needed GM part # 25117765 to make the connection.