Help! 1991 Fuel Injector Help Needed - Rough IDLE

[My84Vette]

Hey everyone... I sold my 84 and got a great deal on a 91 with 33K miles on it, the car is awesome inside and out.. a real dream for me actually.

I need help with a rough idle at a stop light problem. The car runs fantastic on the street or the highway but at a stop light it runs like it has a miss.

I just performed a full tune-up on the car, new plugs, new wires, new distributor cap, new rotor and a new fuel filter but still have the same issue.

I just ohmed the fuel injectors and this is how they ohm out:

16.6
16.5
16.8
15.9
16.7
16.6
16.7
16.6

Can anyone confirm for me that the fuel injector that reads 15.9 is most likely my problem?

Thanks for all of the previouse help on my 84... you guys and this site are awesome...I hope I can return the favor at some point.

Regards!!

T

 

[c4c5specialist]

HI there,

Nope, because GM gives a tolerance of 1.0 ohm from the baseline of 16.5

I would be looking at one thing, how does the vehicle run when ice cold and you start the vehicle? The first minute of run time would be the key.

If it runs great then, but changes when it warms up, you have a fuel delivery change that the computer is compensating for.

If I was to suggest a course of action, a bottle of Chevron Techron to the full tank would be the first thing to try.

Allthebest, c4c5

 

[My84Vette]

HI there,

Nope, because GM gives a tolerance of 1.0 ohm from the baseline of 16.5

I would be looking at one thing, how does the vehicle run when ice cold and you start the vehicle? The first minute of run time would be the key.

If it runs great then, but changes when it warms up, you have a fuel delivery change that the computer is compensating for.

If I was to suggest a course of action, a bottle of Chevron Techron to the full tank would be the first thing to try.

Allthebest, c4c5

Hey C4C5, thats good information, little more info... when I picked up the car, I drove it 200 miles back to my house and did not notice the problem...seems like it popped up after I got home. In thinking about it, it does seem like it started a little after I put gas in it the first time...I did use regular gas, not the highest octane and I do notice some pinging....does this information also point to the Chevron Techron to the full tank and possible to resolution of the rough idle. And yes, at startup from cold, it runs and idles great.

Regards!!

 

[Hib Halverson]

First, don't put 87-octane fuel in your 91. The results will be detonation at part throttle (that pinging) and significant knock retard at high or wide-open throttle. There is little advantage in using low octane fuel. You loose any potential savings via reduced fuel economy due to retarded timing. You also reduce performance and increase coolant temperature.

As "c4c5Specialist" suggests, the idle problem is unlikely an injector issue. To know for sure run, the fuel pressure test, listed in the Factory Service Manual. That will tell you right away if you right away if there is a leaking injector.

A detergent-based injector cleaner, such as Chevron Techron or Red Line Complete Fuel System Cleaner, may solve an idle stability problem, too. Detergent-based cleaners can be used aggressively. I'd use one bottle per tankful of gas for the next three fill-ups then re-evaluate the problem. Where they can help is with dirty intake valves which, under some circumstances can cause misfire at idle. A good detergent based cleaner can help unfoul partially fouled injector nozzles.

Also do the "open/closed loop comparison" suggested by "c4c5Specialist", if the engine runs ok right after cold start and during early warm-up, but then begins to surge, buck or miss, during late-warm-up and when it's at operating temperature, the system maybe struggling to maintain proper AFR at idle in closed loop and that will affect idle stability.

Has the check engine light been on at all?

 

[My84Vette]

Hib, thanks for the info, what is AFR that you are referring to? The check engine light is not on and has not come on at all. The car does seem to idle fine at startup from cold.Your help is greatly appreciated.

 

[Hib Halverson]

Hib, thanks for the info, what is AFR that you are referring to? The check engine light is not on and has not come on at all. The car does seem to idle fine at startup from cold.Your help is greatly appreciated.

AFR= air:fuel ratio.

If the car idles nice during start-up and early warm-up but idles rough or surges during late-warm-up and once it gets to operating temperature, as c4c5Specialist suggested earlier, you've got an engine controls problem, perhaps with fuel delivery.

The Factory Service Manual is the best place to start with a problem like that. If you are not comfortable with engine controls diagnostics on a DIY basis, you may want to look for a service shop familiar with diagnostics of pre-96 GM systems.

Admittedly, it's a bit of a drive from Charlotte but in Bowling Green KY, there's a very good Corvette service shop which can likely solve the problem. See:http://www.automastersofbg.com/services.shtml

 

[c4c5specialist]

Hi there,

Ok, so based on your description, why not start with the basics??

You have already assisted in diagnostics and you stated that cold its fine, warm it is not.

So, that being the case, lets start with FACTS.

1) when your o2 sensor is warm, its starting to tell the computer something that is not helping but HURTING the way the idle is.

2) You can look at many factors but lets look at it simply. Disconnect the o2 sensor and see what happens. You will see a check engine light after a little while, but if the idle stabilizes, you know the sensor is likely skewed, creating the condition.

3) Techron in 20 oz bottle form will treat up to 19 gallons. Add one to a tank and see what happens because it will NEVER hurt your engine.

Lets keep it simple, so that you are not having skyhigh bills just to fix something you can fix yourself.

Allthebest, c4c5

 

[My84Vette]

C4C5, I really appreciate the help you have provided, stopped at Advanced Auto this morning, the 20oz bottle of Chevron Techron was buy one get one free , I ran the tank down to about an eighth of a tank, added the Chevron Techron and filled with "PREMIUM" gas, will run that for a while and see what happens.

If I continue to see the rough idle, I will perform the O2 sensor test and I also have a fuel pressure gage to check that too just to completely rule out the fuel pump.

Thanks again for the help!!

T.

 

[jp1]

Read this service bulletin available here.

Corvette Action Center Knowledgebase :: 1986 - 1991: Service Bulletin: Multec Fuel Injectors / Cleaning Information

I don't know if this applies to over the counter techron. But Multec 1's 89-91 should not have any heavy concentrated cleaner induced into them. 10 percent ethanol destroyed mine at 31000 miles. Not saying you aren't having injector problems as you more then likely are. But wiping out the windings with high concentrations of cleaner are not going to do you any good either. As far as I am concerned multec 1's should be taken out of service for preventive maintenance. If they are not failed they will fail soon. oh, secondly Ohm out your injectors hot that is when they generally show skewed ohms readings.

 

[John Robinson]

You might be interested in going to c4c5 specialist twitter site Twitter and seeing his qualifications.

I am not sure about injector cleaners causing injectors to go bad. But I am sure that time not millage or fuels has a bigger part in there failure.

A 1991 car that only has 31000 miles on it has been in storage a lot and when it was run it was most likely with old gas. Occasionally it was probably filled up but any storage time with a half tank or less of gas would allow moisture into the fuel. Now while the car sits the fuel in places like the injectors etc will slowly evaporate and leave a shellac or glaze behind. Sometimes shade tree myths get started about things like the opti being a bad design or the injectors or whatever. These myths probable came about because someone had a genuine problem and was kind enough to let others know about it. Suddenly it becomes a Oh No dreaded high dollar problem for everyone just automatically assumes that when there car hiccups it has to be the dreaded whatever. In the case of the opti I two was hearing about all the evils of the design and yes mine finally failed after being in the car from 1993 until 2008 and it showed signs of corrosion caused by moisture. When I bought my car in 2005 it only had 67000 miles on it so it to was a car that had sat a lot and collected moisture in places like the opti from a lack of use. Please understand the injector re builder is probaly right in his assessment of the injectors for that time period. He is a professional and knows his trade. My meager experience with low mileage engines and fuels comes from resurrecting long term stored motorcycles. Normally the best thing that can happen is too get them up to temperature and run them for an extended period of time. When I got my 93 the first thing I did was too drive it a couple of days to make sure it was reliable and then I drove it one day for 500 miles so I could run a couple of tanks of gas in it. About the time I hit 300 miles and filled it up it ran like a clock.

 

[jp1]

Obviously Mr Robinson you did not go to the link I posted. This is a gm service bulletin not a myth. This info only applies to multec1's in 89-91 cars. not your lt1 car. The insulation of the coil of the multec1 injector is not tolerant to solvents or alcohol. It will cause the insulation to dissolve and cause the injector to short out. Gm put out that bulletin due to the injectors being destroyed at the dealers. I posted my original post as a warning. What people do with their cars is their choice.

 

[John Robinson]

Obviously Mr Robertson you did not go to the link I posted. This is a gm service bulletin not a myth. This info only applies to multec1's in 89-91 cars. not your lt1 car. The insulation of the coil of the multec1 injector is not tolerant to solvents or alcohol. It will cause the insulation to dissolve and cause the injector to short out. Gm put out that bulletin due to the injectors being destroyed at the dealers. I posted my original post as a warning. What people do with their cars is their choice.

First my name is ROBINSON and I was maybe in error with my response but looking at the number of post you have made I thought you were a newcomer and perhaps were unaware if the extensive qualifications of c4c5 specialist. Some of us remember the hard work and extra schooling he went through to get that rating on Corvettes. For that reason I tend to believe what he says and I think he was saying that using that product one time could solve the problem but if not what is the harm because then the injectors are bad and need replacing anyway. I will stand by my suggestion that low mileage cars have problems brought about by a lack of being run enough to properly bring everything up to temp and then stored without any thought to what humidity in the air will do to the electrical parts of the car. It is my sincere wish that we go forward on the post with the intent to help solve the posters problem.


In rereading the OP I noticed that this is temp related and that a fuel delivery problem may be the culprit. Just recently we had a post by someone who had found all kinds of rust and crud in the gas tank of there low mileage car. If that has happened to this car (only an inspection of the tank would show how bad it might be) then it is possible that the fuel filter or the fuel pump pick up filter is getting full of crud and causing a restriction in fuel flow.

 

[TedC]

Let's try and remain professional here.

I don't think a few bottles of Techron is going to do any more damage than that already caused by ethanol in the gasoline. If these are Multecs, then the coils are exposed to raw fuel for cooling, and due this car sitting around with such low mileage, it can't be a good thing for the fuel delivery system (tank to injectors).

Low octane fuel should not cause rough idle. Mine idles the same on 89, 91 or 93. Yes, the L98 is optimized to run on higher octane fuel.

So it ran fine, then started to run rough after a fill-up. Do we have fuel contamination? Crap in the gasoline from the station, or already in the tank, or injectors not atomizing properly? A static resistance test is only a partial indication of injector health. Need to test cold and hot because heat causes the internal coils to expand and may show a short that was not visible before. This static test does not help tell us about proper fuel flow.

I suggest checking the fuel pressure level at the rails (engine off and engine at idle). Use a pressure gauge that has a bleed valve and hose so you can purge a pint or two of the fuel into a glass container. Is it crystal clear?

Longer term, replacing the Multecs with Bosch IIIs is a smart investment. But first we need to pinpoint the source of this rough idle. We sure don't want to plug up the new injectors.

 

[John Robinson]

Take a look at this thread. In the 4th post there is a picture of the insides of a low mileage 91



[h=2]

Ohhhh! NOOOOO! I have to remove the Tank on my 1984[/h]
This is a graphic example of what condensation over time will do. This is also a poster child for not storing the car with anything but a full tank to keep the air out of the tank. At some point and time when you have run this great car for a little bit you may want to replace the filters in the fuel line and the fuel pump pick up screen.

 

[My84Vette]

Hi Ted,

I agree on remaining professional , my post was just to get a little help with an annoyance on the car. I am a shade tree mechanic and hobbyist, by no means am I a professional, thats why I am here.

The rough idle only comes to play when at a stop light or at a stop when the engine is at operating temperature, it kind of feels like a miss but the car does seem to buck a little. When cold, the motor runs very well and at speed it runs very well, dont feel a miss at all, if this was a fuel delivery problem wouldnt I feel it at all times?

One more thing to note, when coming to idle from highway speed I do notice what feels like a stumble.

Much appreciation to all of you for the help on this.

 

[TedC]

Hey, I'm a Senior Shade Tree Mechanic. My vette is one of my hobbies. otfl

As pointed out above, you need to check the quality of the fuel being delivered to the injectors. A car that sits around is NOT good. Injectors are turned on and off very quickly by the ECM. At closed loop when the engine is hot, the switching (pulse rate) is at the lowest levels (especially at idle). So it is possible to have a closed loop idle issue that goes away off idle. I would want to confirm that the fuel deliverly system is AOK. Good time to invest in the FSM.

On all the L98s I've worked on, two being my own, I've found how critical the throttle body is to a smooth idle and elimination of any stalling or hint of stalling in closed loop. IAC, TPS, and setting the min air speed properly made a significant difference (more than even a tune-up). Next was making sure the fuel is clean and the injectors are working properly.

 

[My84Vette]

Thanks Ted, I drove the car into work today (40 miles) after filling the tank and adding the Techron, seems to be idling smoother..dont know if its wishful thinking or not, but I will run this tank through and see what happens.

I will put the fuel pressure gage on before the weekend to see if there are any issues there... I did check for stored codes in the ECM last night.. and that came up empty as well.

I will invest in the FSM...wish I could find someone that needs the FSM from my 84 though... bought the manual and then sold the car :L. I plan on keeping this car though... not discouraged at all..just want to get it running perfect.

 

[My84Vette]

Question regarding IAC?

Still chasing this idling issue, running the Techron through seems to have helped a little, I have see a lot of information on cleaning the IAC valve.

Is this reccomended? If so, has anyone seen a step by step on this? If I take it out to clean it, will it need to be re-adjusted?

Just curious...

T

 

[aklim]

Still chasing this idling issue, running the Techron through seems to have helped a little, I have see a lot of information on cleaning the IAC valve.

Is this reccomended? If so, has anyone seen a step by step on this? If I take it out to clean it, will it need to be re-adjusted?

Just curious...

T

I don't believe in the snake oils. I have never seen more than anecdotal "evidence" that it works. Ohming the injectors really tells me that the electronics are toast or not. Does not tell me about the spray pattern. Snake oil basically might clean it, might not. When you can come back and show me laboratory tests that prove efficacy, I believe it. By laboratory tests, I mean something that was done in a reputable lab using proven test methods and you publish them. If it is something you and your brother did in the basement, well, as far as I am concerned, it is about as good as the proof that Enzyte and Extenze provides. Proof that people are stupid enough to part with their money. If your injectors are old, I would suggest getting them tested and maybe changed to the later designs besides Multecs. Without a flow bench, all you have is a SOTP dyno "test" which, IMO is about as good as a SWAG. It might not cure your condition but it is something that, IMO, should be done every so often to clean up build up. Before you ask, NO, I do NOT work for any flow testing agency.

Cleaning the IAC valve is something that should have been done every so often. Why? Because build up occurs. Just like in the injectors. The creep is gradual and very hard to notice. In fact, until it gets out of whack, you are going to be hard pressed to notice it. The way I would approach it is to take the TB off and remove the IAC valve and the IAC housing. Since I don't reuse condoms or gaskets, I would get a new set. Clean the whole thing up with brake cleaner. I would re-adjust it just to be sure. If you have access to a scanner, it isn't too difficult. If not AND your system is stock, I think there is a GM procedure to adjust it.

 

[TedC]

You will want to remove the TB to properly clean the passages and back side. I would look at the TB for the year to make sure some PO didn't swap it with a different year. I worked on one L98 that idled like crap. Come to discover it had the wrong IAC (three different designs). I don't waste my time cleaning the old IAC anymore, I just replace with a new one (comes with new gasket and proper thread sealer). You need to make sure the pintle is pushed in enough to avoid contact when installing. Check out one of my old posts for more information. http://www.corvetteactioncenter.com...ottle-body-adjustments-1989-l98-corvette.html