1) Can the computer through codes without the Check engine light being activated?
2) Mine is running rough after warmup (closed loop?). It's not missing but it feels similar. It's surging a little which can be felt more during decel while in gear.
3) It smells rich
4) New Bosch O2 sensor last year with new exhaust.
5) No check engine light in last several weeks. Problem started recently.
6) New plugs about 1 1/2 yrs ago (10K or less on plugs).
7) Popping at little on decel -- especially during open loop.

My feeble thoughts:
1) Pull plugs and inspect (PIA)
2) look for vacuum leak
3) Rap on maf sensor with screw driver (already did it)
4) Replace O2 sensor
5) take for diagnosis at chevy dealer
6) sell it

 

I have a '90 which had almost the identical problem, two items cured my problem 1.) new 02 sensor, 2.) eight new Bosch 24# injectors.
I too was not recieving codes, nothing, ran rich, almost stalled, and baulked (almost a backfire) when running between 40-45 mph.

Hope this helps

 

It doesn't take long for a code to be set, but I think the CE light will illuminate, whether you noticed it or not. Checking codes in a no brainer, so why ask? Just do it. Your symptoms are possible without a code being set, so follow the trouble shooting charts in your FSM for your symptoms.1.) That isn't the problem, but it's never a bad idea.
2.) Always!
3.) OK.
4.) Don't throw money at it without trouble shooting.
5.) See number the number 4 answer. You won't even get a kiss from the dealer.
6.) As bad an idea that is, it's better than #5.

RACE ON!!!

 

Your profile indicates you have an 89.
If you have the Multec injectors (grey body) unplug the electrical connectors and check the resistance of the fuel injector coils. When cold and when the engine is at operating temp. Should be 17 to 16 ohms.

 

THOSE FIRST GEN MULTI-TECH are a bummer. i have an 89 iroc and with only 17000 on the clock they were junk, 7 out of 8 on the hot diagnostic. i steer clear of rochester injectors.

 

what you describe sounds a lot like exessive spark plug wire resistance.
use an ohm meter, take off the distributor cover, pop off the wire retainer off the distributor, and one by one, pull the wires off the distributor, and the respective spark plug, one at a time, then use an ohm meter, measure the spark plug wire resistance.
reconnect both ends before moving on to the next plug wire.
If the spark plug wire resistance is higher than 1200 ohms, you need a new set.
when you buy new ones, use a high quality brand like taylor that has about 150 ohms per foot. that way, you shouldn't have any more than 500 ohms per wire. it will run a lot better

 

as this is also very easy to check for good/bad injectors. Spec is 16.5 +/-.3 ohms and all eight should be close to the same value. The GM Rochester Multec injectors were used in Corvette years 89 thru 91

 

I have a new set of wires and cap I need to install. That will come next. (I should have put that in my first post).

I have been curious about injectors too. And, posts about them have been helpful. I remember that injectors were listed as a weak item (along with intake manifold seal) when I read up on 89 vettes in 1999. But, I didn't recall how injectors fail.

If their resistance is off, can they inject too much/little fuel? I assume atomization is important as well. I had leaky injectors on other cars, but they're failure was pretty obvious.

I will say the problem does feel like fuel/vacuum. I think it's been coming on for quite a while and has been intermittent. Now the poor idling/smoothness seems more pronounced.

I used to wonder about the MAF cause it thru codes every few months back in the early years of ownership. But, then I read about burn-off relays and changed my startup procedure to allow more time for the computer. I haven't had a code in recent years.

I wondered about the O2 sensor, but replaced it last year. Still, I heard Bosch's weren't great and a dealership scan last year said it was marginal -- but OK.

The engine temp sensor and the idle position sensor are the only other sensors that may/may not throw things off. The temp sensor -- if off -- may be confusing the mixture, right?

Other than that, ignition parts, vacuum, and fuel seem to be the areas I need to look at. I don't see/hear a vacuum leak, wires are next, and you guys were helpful about what to look for on the injectors.

gp

 

An injector with shorted turns in it's coil will introduce LESS fuel into that cylinder. The O2 sensor is located on the drivers side of the exhaust system and the info from this sensor will attempt to control the pulse width of the passenger side injectors too. The car will seem to run OK in the open mode as the O2 sensor is not active at this time. Once the O2 sensor takes control, the shorted injector turns create a problem in operation. Make sense???

 

Because the car is old, the injectors are old, and the plastic parts of the injectors are old, I want to be sure how to unplug/unclip the injector -- to avoid breaking a plastic ear, etc....

Can you describe what to pull/push in what manner to unplug?

Thx!

 

To remove the injector electrical plug, oress the metal spring clip inward either with your finger or a flat blade screwdriver and at the same time pull the connector up and off of the fuel injector.

Then use an ohm meter and place one probe lead on each injector contact.

 

When you check the injectors, you need to do it twice. First when cold, and then again when warm to hot. Sometimes the cold values will be within spec and the hot will not. They have to be in spec when hot as well as cold.

If you do have a bad or marginal injector, reputable injector servicers will not rebuild the 89 Multecs due to their initial poor build quality. The 89 injectors were rated at 22#/hr @ 43psi. Ford Motorsport rates their injector flow at 36psi, so you could replace yours with FMS 19# injectors, or use another brand of 22# @ 43psi injectors. Based on the price, you'd think the GM replacements were CNC machined from $olid gold.

 

In 89, Chevy eliminated the cold-start valve. in open loop cold, the computer chip is programmed for enrichening the injectors to compensate for the missing valve.
If the temp sender to the ecm was no good, the computer would think it's always cold, and dump in cold enrichment fuel....might be worthwhile in ohming the ecm temp sender (front of the intake manifold)to see if it looses resistance between a hot or cold engine.
more resistance cold than hot.

 

I like that suggestion. Isn't the temp sender between #1 and #3 plugs? Is the oil sending unit by #8? Any better ways to diagnos a bad temp sender?

BTW: Plug wires helped alot. The insulation had cracked/burned thru where they round the back of the valve covers.

And dauuummmbbbb. It took quite a while to wrestle those wires out and replace the distributor/wires. It ain't your dad's FORD! I think it about 2 1/2 hours for the wires. Course I finally just yanked em out cause of the looms below/behind the distributor.

Also, 2 of my 8 injectors didn't measure at 16 ohms. #3's measured 14 ohms and #5's measured 12 ohms. When do I need to worry about that?

I also think my thermo is going because it won't hold temp when cold outside. For example, on the highway below 50 degrees, it will drop to about 155. Guess everything starts to malfunction when it gets 18 yrs old! (55k miles on engine/car).

gp

P.S. I broke most of the wire looms and just used zip ties to hold the wires in place. I'm thinking new injectors, thermostat, and valve cover gaskets might need to go in over the winter. So I didn't get pretty putting it back together. Hope I don't start thinking miniram when I get it all apart!

 

Your pic and description are much appreciated! It made things much easier after seeing this post. I'm sure I avoided a broked plastic plug because of it too!

gp

 

I've got two injectors with partially shorted coils it seems. (12 ohms and 14 ohms on #5 and #3 respectively). If they shoot less fuel -- as described above -- and the O2 sensor is on that side, I bet that's why it smells rich. The O2 sensor thinks the injectors -- especially on its own bank -- are running lean. So, it's increasing fuel. On the better-performing passenger cylinder bank, it would be running richer than the driver's side. Right?

gp

 

You are correct. If the O2 sensor registers your need for a longer pulse (open) time due to the driver side #3 & #5 injector problems, the ECM will add pulse time and more fuel in both banks. This causes the passenger bank that was OK to become too rich. The single, driver side, O2 sensor does not report the problem, because it physically cannot see the passenger side bank exhaust. Even with the O2 sensor info working for the the driver side, it will be off as well, because the #1 and #7 injectors do not need the increase in fuel any more than the passenger side injectors. Because of this, later model C4s got two or even 4 O2 sensors and sequential injection. OBD2 became mandatory in 96, and it requires 4 O2 sensors.

 

Thank you ..... Mike! Well said and this "concept" took me some time and loss of gray hair to understand!

 

GeggPen, unfortunately you are overlooking something else that will give you the "rich condition" and that is faulty AIR solenoids. The AIR system can introduce air into the exhaust the O2 sensor reports to the ECM that there is a "lean" condition and the ECM opens the injectors.

Tom

 

Thanks for the complement Sam Lam. Yes indeed, on the single O2 sensor C4s, the system can very easily be doing the wrong thing for the right reasons and fool us big-time. If GREGGPENN's problem is what it appears to be, the side that is flowing the richest, is probably the side that has no bad injectors.

I agree with tdr1919, that there coud be other parts issues besides just the 2 injectors that are reading at the edge of spec. That area should be checked/tested as well. One bad part isn't always the total cause of the problem.

In this particular case, the car is an 89 with what appear to be the original injectors and the resistance test has shown 2 of those injectors to be marginal to bad. Even the best injectors eventually need rebuilding or replacement after years of use. The 89 factory multecs are known to have been of marginal quality when they were new. After 18 years, I doubt if they "got better", and I think it is pretty safe to say they need to go. I would replace all 8 injectors as a maintenance starting point. You can do the job yourself in a weekend and save a bunch of money.