Rough Idle - 92 Q45

[MarkQ45]

I am experiencing rough idleing between 600 - 750 rpm. (when warm) I have had fuel injectors replaced before which helped when it was real bad. Any suggestions? Can someone give me instructions for testing and replacing fuel injectors?

[Q451990]

This may be related to your fuel pump problem over in the other post. Also, injectors need flushing from time to time. Replace the pump first - then have a powerbalance test done for a baseline. The plenum (especially EGR port), EGR tube, throttle body, runners, etc. all need to be cleaned to like new condition. Then have the injectors flushed with a pressurized rail flush (I prefer BG products). After the "blow the doors off" WOT runs for 10-15 minutes to remove any crud that accumulated on your valves and pistons - try another power balance test. You should have no more than 12 RPM variance between cylinders if all is well. Also, a loaded test will tell you more (car in drive against brake w/ A/C on at 2000 RPM). Oh, and if the spark plugs have over 60K, I'd replace them too, preferably after all of the cleaning so you don't take a chance of baking crud on new plugs.

All of this can be very expensive, but that's what it takes for a smooth idle.

Heath

[hobber]

I have a '94 Q with 81K miles and I just started getting a rough idle today. I took it into the shop and they said one of my injectors (#3) is bad. Will a fuel system flush help this problem or will I have to replace the injectors? I have also noticed my fuel pump is humming. Do you usually have to replace the pump or I had heard that you can just replace the modulator?

Thanks.

[PalmerWMD]

1) Are your plugs original? If so they need to be replaced (this is not as cheap as many think, if any of your other jobs get close to the plugs just do them for good measure, even if they are under 60k)

2) Sadly replacing injectors can be very expensive also. Some injectors are easy to get to=less labor; others are hard to get to=lot's more labor.Often once one goes bad, the others quickly follow, when I had 3 injectors bad 1 year ago and one marginal I told them just to replace all 8 to save labor on future replacments.

3) sometimes a BG44k injector rail flush will help things. and postpone the need for replacement.remember to replace the fuel filter after a injector flush.

4) It is always cheaper to replace a humming fuel pump before it takes the (more expensive) regulator out.

Fred...

[DAEDALUS]

The #3 injector is a difficult one (3-6 are under the plenum). Regulators are pretty durable...I think Fred meant to replace the pump before it takes out the electronic controller module. I just bought a pump on Monday from infinitipartsusa.com and it was around $235. The controller is well over $300. Still, don't replace anything without a little more confirmation. You can buy a pressure gauge for about $30, and it only takes about 20 minutes to hook everything up. Pressure should be 43.5psi min.

[PalmerWMD]

Regulators are pretty durable...I think Fred meant to replace the pump before it takes out the electronic controller module. .

That's exactly what I meant.

Fred....:oface

[Q45tech]

Actually something on the order of 50 psi for a short period is good since the injector sees 34 psi most of the time and rising to 43 psi at WOT. The system is capable of handling 60 psi. on really bad units I double the 11 ounces and run the flush for 30 minutes then with 44K in fuel, drive the car agressively [load and rpm for another 15 minutes...............all depends on the before after power balance tests.

Unless there is a cylinder leak from really bad rings or valves or a greater than 18 psi compression imbalance the Q should idle like new even after 240,000 miles..........mine does but it requires some work [attention---at least 44k] every 90 days [as there is a little valve stem seal leak which does allow a little oil to suck into the cylinders until the engine warms to operating temperature.

The system [fuel and air] should be cleaned until you have a repeatable power balance test that shows no variation between cylinders [well not more than 12 rpm at the worst case].

Obviously any idle shake comes from one or more cylinders not making its full power by the 1200 rpm of the balance test, it can safely be inferred that this condition extends up the rpm band!

After your plugs have 4 years or 60k on them it maybe that they are less than optimum especially if the vehicle is used in city traffic without the aggressive WOT weekly/monthly cleanout expected of high performance engines.

[firstq]

I read again and again on the power balance test - is a consult required for this or is it a DIY item doable at home as well?

Trying to tell the maximum allowed 12RPM difference w/o a machine [consult] sounds almost impossible...

I was interested in doing a fuel rail flush, but not sure how to verify the condition before and after at home.

Thanks

[DenverQ]

Dennis do you ever take the injectors out and bench clean them or pressure clean them off of the car like when you pull the plenum? Or do you just do the normal injector flush every time

[DAEDALUS]

The power balance test can be done at home but it is as difficult and more time consuming than replacing the plugs. You will need some method of measuring RPMs (the dash gauge simply will not do). Automotive multimeters with an inductive sensor can do this, but they're not in everyone's toolbox. You then need to disconnect each plug one at a time to measure the RPM drop. If you have no other reason to disassemble things in the engine bay, you might want to get a quote from a dealer.

[Q45tech]

The Consult automatic power balance test raises the idle rpm to 1000-1200 by locking the IAC open and disabling the other portions of idle speed control [ignition advance injector duty cycle, etc] then progressively kills an injector and associated spark for around 3 seconds and averages the rpm drop to the nearest 12 rpm [50,62,75,87,100] impossible to do accurately manually because you can't get ecu to stop trying to control rpm......the base idle will change as you disconnect an injector!

A BG product not BG44k is used as a rail flush the 44k won't support combustion properly!I have a special cocktail [a dash of this and that] but 90% the BG Injector Cleaner in a Package which includes a 44k for the tank.

GM test have shown that even 15- 20 minutes [amount of time for 11 ounces to burn at idle] may not be long enough for the solvents to work so I add a little gasoline and 44k to make 15 oz or 30 minutes worth.

On mine I double fill [22 oz] and let it go 40 minutes at 44-50 psi to help force the crud thru the injector filters.

I have been keeping close records on my idle rpm and have found that it decreases by 12-25 [3-4%] after only 4,000 miles from throttle body crud [from ring and valve stem seals under high vacuum cruise/idle].........to be expected at 240k.

Seems normal [and progressive] as a 60k engine need cleaning every 15k. But the factory minimal recomendation is once at 30k and 60k.

BG44k is designed to keep injectors clean not clean up dirty injectors. If you use it every 3 months it might do something towards clean up but 160 gallons [3,000miles] is a lot to ask of 11 ounces.....1 part in 2,000.

Probably 90% of injected cars on the road are not near oem specs of a 5% flow balance at idle and a 12 rpm variance is 1.8% so oem spec would be a 32-37 rpm vaiance. But they never tell you this in any manual.

Hint the Consult reads "OK" if the cylinder has any rpm drop.....tells you its working....somewhat so ignore the "OK" and zero in on the actual variance.

[Q45tech]

If the maximium compression variance is 14 psi out of 185 or 7.5% and 4 cylinders per rpm fire ........the rpm variance should be no more than 1.9% or 12 rpm from compression alone.

We throw away used injectors as cleaning doesn't help repair the damage done by fuels.We have had serious problems when injectors fail [from fuel leaks[hydrolocking]] nobody in right mind would take a chance on replacing an engine to save a few bucks on injectors. Businesses cannot afford to gamble saving $400 against a $6,000 motor doesn't make sense to me.

But then again few shops have replaced Q motors, after you do a few dozen you get paranoid about everything.

[DenverQ]

I have to goto Lincoln Nebraska next week, I think I am going to run 44k the way up and then on the way back. Wonder what its going to be like on 93 octane and only 1100ft I can only imagine the power

[Q45tech]

5500 minus 1100= 4400/1000 x 3.3=14.5% more than Denver you need every point of octane to compensate for the lower altitude!

The problem with even high speed cruise is the injectors only see 34.4 psi unless you are accelerating so the normal pressure doesn't exert much cleaning action!......especially going downhill all the way

[DenverQ]

Hmm good point I might as well get the injector flush then, but now here is the Question would it be better to get it before or after the trip?

[DAEDALUS]

You wanted to enjoy the car's performance at the lower altitude, right? Get it done before the trip.