64Scout

This may end up being a long post because sometimes I get scatterbrained. But here goes...
I bought an 89 6 speed about a month ago everything was good on the test drive, a few smafew issues but nothing I can handle or live with. After I got it home and started driving to work and back, it began to act up. What happens is at some speeds it's like someone turns the key off. So what I would do is let off the throttle all the way for a couple seconds and get back on. My thought was that maybe the fuel pickup wasn't submerged and by getting off the gas it had time to settle and srop sloshing. Well I was at about 1/4 tank, so I filled it up and it did it a couple more times on the way home from work, then didn't do it anymore. Now it's been happening again, this time I just lift my foot a hair and the engine comes back on like a switch. Again at 1/4 tank. I think that maybe possibly it's due to the throttle position sensor. The SES light is on, but I haven't had time to make a tool (bend up a paperclip) and read codes. Also, and maybe related, sometimes the engine idles at 1600. I've checked to make sure the throttle was closed all the way, so I'm thinking this is the idle air control? Any thoughts, aside from pull codes first?

Tom400CFI

My thoughts are that since you have a code, that would be step one. It probably took you longer to make your post than it would have to make the bent paperclip.

ctmccloskey

Hello there 64scout,
First, get a code reader and determine what the codes are for. It sounds like you have multiple issues that are affecting your Corvette.

These cars use an electric fuel pump and in that circuit is a relay, this particular relay on my 1988 C4 L98 is above the steering shaft on the drivers side mounted on the firewall. It is the same relay that controls the electric fans and probably other places knowing GM. I keep a spare in my car. The relays are easy to test and were less than $20 the last time I bought one.

Random cutting out means the first thing you should do is get a Fuel pressure gauge and connect it to the valve on the passenger side fuel rail. The take the gauge and tape it to the windshield so you can see it while driving. Drive the car and when it starts cutting out check your fuel pressure. You might have something as simple as a clogged fuel filter or possibly a fuel pump getting ready to stop altogether. If your filter was plugged up it would do what you are describing as well so if that has not been done in a while. A Corvette with a 1/4 tank of gas can cavitate the fuel pump periodically. I try not drive on the bottom quarter as this is a issue we have to deal with. I was taught to keep the fuel tank full to keep condensation from forming inside the fuel tank. On your car does it ever do the "cutoff thing" with a full tank or half a tank?

The idle going to 1600 sounds like the car might have either a vacuum leak or a bad temperature sensor. When does it do the 1600 rpm issue? When the temperature sensors produce wrong data then the car will think it is either too cold or too hot and this really affect the starting system. You would not want the engine to think it is 30 degrees when in fact it is 70 degrees, there are two temperature sensors on my 1988, probably on yours as well. One is for the gauge on the dash and that one is a basic sensor that gives a ohm value for the temperature. The second temperature sensor uses a five volt reference signal and determines the engines operating temperature, the data from this sensor goes to the ECM. We have seen a lot of bad sensors here at the Forum and they cause all kinds of starting issues. With the 9th injector (Cold Start Enrichment Injector) if the system is activated when it is warm you get a flooded engine if it does not work at all you get major cold starting problems.

The other item that would make the ignition shut off and turn on randomly would be the ignition control module, when they start to fail frequently they display random shutoffs after the module has warmed up completely. Again the test procedure is in the Factory Service manual

How many miles are on this Corvette? Has anybody done anything to modify this car or is it still basically stock? Do you have a set of the Factory Service Manuals for your particular year Corvette? They are VERY helpful to have if you plan on keeping the car a while.

Whenever I buy a used car I always like to change the belts, fuel filter, air filter, oil, oil filter, coolant, power steering fluid and brake fluid. This way I 'Know" what condition they are in. One product I really like on my 1988 Corvette is BG 44-K fuel injection cleaner, I run a can through every 4-5000 miles and it keeps my car running very smoothly.

Here are some ideas to work from, I hope that they help you!

64Scout

Yeah I posted while at work. I work 12 hour days. It's dark when I leave for work and dark when I get home. Pulling codes has to wait for a day off.

64Scout

yeah it's just got to wait for a day off work. In the meantime , I can ask and do research.

Yes it has done it with a full tank. Sometimes while cornering, sometimes while in a straight line. It doesn't come back until I let off the throttle which is why I dont think its fuel related. In the meantime I'll put my fuel gauge on and see if it does it again. If it was fuel, wouldn't it sputter a bit first as the pressure drops?

it too has been random. Less random that the cutting out. Dash shows temps in the 180's, but that doesn't help if the ecm sensor is out.

The car was advertised as having under 70,000 miles, but I found that the odometer didn't work, so who knows. The guy I bought it from only had it two or three months, he had some stuff done like a clutch and I dont know what all, but aside from the mystery switch under the dash and add on gauges it seems pretty stock. And now that i think about it, one of those gauges is a temp sensor, so i should probably track down where it goes to on the motor and see if it is T'd to another sensor or replaced a sensor.

64Scout

Finally got some time to pull codes, and here they are.
22 TPS
33 MAP
36 MAF
45 O2

So yeah, a little bit of work to do, see if that fixes things.

Plus I've got this doodad plug here that I can't find what it goes to, and no I dont have the ninth injector.

64Scout

Side question now that I think about it, the engine should either have MAP or MAF, but not both, correct? Looking g at another chart I see this and since the car is manual (wouldn't have that trans anyway if it wasnt] maybe I'm mistaken thinking the code is for a mass air sensor? I wasnt aware the tpi used a CPS, I mean I have a 305 tpi running a tbi ecm, and it just runs a big cap hei, unless the sensor is in the distributor.

kael

Going by my '87 FSM, 33 is MAF. 36 is also MAF, but this is the burn circuit, it happens at engine shutdown after closed loop, then lights the next start if the burn failed. That code is a big PITA, search the forums and you'll find plenty of threads.

33 per '87 FSM is excessive air flow, more than 45 grams per second.

Now, again, this is for an '87. There are significant changes in the '88, not sure if the engine was changed all that much. I do recall a switch from MAF to MAP at some point, possible that was the '88 and if so, the codes I described don't match.

Another bit to check, this could cause random problems, your spark plug cables might be bad or cracked.

64Scout

Plug wires look pretty new, but whoever put them on twisted them around each other. It's not running rough or misfiring other than the random loss of power. Maybe tomorrow I cam check the fuel pressure if my wife will watch the kids for a bit.