99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

[dr_lee29]

One more question, if the injector at cylinder #4 is bad, should I just change that injector or opt to change the whole spider? Its expensive but I don't want to keep doing this every weekend.

[MT-2500]

Sorry.
We can not tell from the pictures.
What are you going to do now?

You should have done some proper fuel pressure testing and a injector balance leak down test first.


Good luck

[dr_lee29]

Yes I know I should have done some fuel pressure testing but I don't have the proper gauges to do those tests. I thought that perhaps if there was a leak on one of the fuel lines I could see the gas on the manifold. Is the spider always wet like that or is that normal?

I cleaned the tips of the injectors and put the thing back together. It still runs the same, misfire on cylinder #4 SES light.

I'll have to take it to a mechanic because I don't have the gauges to do the rest of the diagnostics and by the time I buy all the gauges I'll have spent more than the mechanic fees. I've been taken by a few mechanics over the course of owning this truck so I'm reluctant to visit them.

[dr_lee29]

By the way, what should I expect in terms of diagnostic fees? Can I get an estimate up front?

[MT-2500]

Originally Posted by dr_lee29

Yes I know I should have done some fuel pressure testing but I don't have the proper gauges to do those tests. I thought that perhaps if there was a leak on one of the fuel lines I could see the gas on the manifold. Is the spider always wet like that or is that normal?

I cleaned the tips of the injectors and put the thing back together. It still runs the same, misfire on cylinder #4 SES light.

I'll have to take it to a mechanic because I don't have the gauges to do the rest of the diagnostics and by the time I buy all the gauges I'll have spent more than the mechanic fees. I've been taken by a few mechanics over the course of owning this truck so I'm reluctant to visit them.

The injector unit and manifold is always black and nasty looking.
If there is a external leak it will be bright and shinny around the leaking spot.

When the upper intake is off I always hook up the fuel lines and prime up fuel pressure and watch for leakage.

Leakage will show up with a fast leak down on fuel pressure test.

Find a good repair shop and have them test it out.
A good repair shop will give you a estimate of diagnostic cost.
A real good test out can run into a hr or more labor.
Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.

[dr_lee29]

Took the truck to the mechanic yesterday. After an hour of looking they called me and said they still can't figure it out and wanted to know if they could keep looking. I told them they could. How long should I let them go before I give up on them?
One thing they said is that the truck only misses at idle, at 1500 RPM the miss goes away. I agree, the truck seems to run fine on the road, the idle is bad and the gas mileage is low, not to mention it never gets warm. They were wondering about the distributor? Any thoughts?

[MT-2500]

Originally Posted by dr_lee29

Took the truck to the mechanic yesterday. After an hour of looking they called me and said they still can't figure it out and wanted to know if they could keep looking. I told them they could. How long should I let them go before I give up on them?
One thing they said is that the truck only misses at idle, at 1500 RPM the miss goes away. I agree, the truck seems to run fine on the road, the idle is bad and the gas mileage is low, not to mention it never gets warm. They were wondering about the distributor? Any thoughts?

That is hard to say.!!!!!

Did you check them out?
Have they got good trained techs?
How much equipment have they got to test it.
How much are they charging?

What test have they run on it?

[dr_lee29]

Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?

[dr_lee29]

Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?

[dr_lee29]

I looked at the vacuum on the engine and it looked fine to me, 19 steady at idle, about 21 steady at 2000 RPM, dropped with accel, raised with decel.

[MT-2500]

Originally Posted by dr_lee29

Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?

All of that does not sound quiet right.

Did you check them out first?
Have they got trained techs?

Maf sensor can be cleaned with a shot of MAF sensor cleaner or just unplugged for test.

Was fuel pressure tested good?

Have you got the temp back up to 195?

Any missfire codes?

Vacuum redings are good.
A injector balance test would be a good test to run.

[dr_lee29]

I'm new to this area but I did check out the group. They are certified mechanics and suppose to be specialists in diagnostics. They were the only garage in town without a bunch of negative comments posted online.

I replaced the injector on cylinder #4, so I know at least that injector is good. I'm going to pick up the truck tonight without letting them do their suggested repairs. I guess I'll bite the bullet and buy a fuel pressure gauge and probably a distributor cap (can't hurt anything its part of a good tune up). But I think I just lost the $150 in diagnostics fees.

[MT-2500]

Originally Posted by dr_lee29

I'm new to this area but I did check out the group. They are certified mechanics and suppose to be specialists in diagnostics. They were the only garage in town without a bunch of negative comments posted online.

I replaced the injector on cylinder #4, so I know at least that injector is good. I'm going to pick up the truck tonight without letting them do their suggested repairs. I guess I'll bite the bullet and buy a fuel pressure gauge and probably a distributor cap (can't hurt anything its part of a good tune up). But I think I just lost the $150 in diagnostics fees.

150$ in diagnostic fee should have found the problem.
Did they say for sure the MAF sensor and or dist and vacuum leak was the problem?
Has the temp engine running problem been fixed?
Is online the only place you checked them out?

[dr_lee29]

Picked up the truck, $170 for diagnostics. I have a list of the tests that they ran.
The fuel pressure test says 54 psi no drop after 10 min. This sounds low to me, I expected 60?
The IAT failed the smoke test. (I put a new IAT in just a few weeks ago).
10" vac at PCV return hose
Bank 2 fuel trim always neg even at 2400 rpm
#4 compression 130 Dry
#4 running compression 0-50 fluctuates
Miss goes away after 2300 rpm

[MT-2500]

HAS THE ENGINE RUNNING TEMPT BEEN FIXED?

I always check engine vacuum at a main engine source.
Like the brake booster hose.

Uneven fuel trims could be caused by engine miss.
Also 02 sensors.
On some engines I have had a 02 sensor throw a miss on no 4 cylinder.
Back in 95 trucks they had a 02 sensor wiring problem that would cause a cylinder 4 miss.
There was a TSB on it that required a 02 sensor wiring replacement.

Yes you are right on fuel pressure.
LOW FUEL PRESSURE WILL GIVE ALL KINDS OF PROBLEMS.

You need to get the fuel pressure up to par to start with.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.