I just picked up a 1999 C5 convertible with 39k on the clock. First vette for me and first at troubleshooting a LS type engine. I didn't really notice the rough idle when I test drove it, maybe it was the convertible and the really nice day, either way I have to figure this out now. I know it should be idleing much smoother based on seeing others at the local corvette club.

Here are the stats:
1999 C5 with 39 k miles. All stock except the previous owner(s) put in aftermarket rockers and springs. There are witness marks on the inside of one valve covers where the previous rockers hit after a trunion must have broke. From looking at the rest of the bolts in the engine bay this looks like the only thing that was done to it.

After reading many many posts and trying things I can not figure the rough idle out. I have a scan tool and able to log data but just need a second opinion. Things tried are:
New Spark plugs and wires
Checked for broken valve springs, none
Checked for vacuum leaks many times, had the intake off looking at the back vacuum lines, all looked good
Cleaned injectors and verified spray pattern
Fuel pressure ie 58 psi at idle and 2500 rpm +
Cleaned MAF then ended up getting a new one
Checked/cleaned engine bay grounds, nothing looked bad
Compression test, all cylinders within a few pounds of 210 psi
Used old school vacuum gauge, all looked good except was reading low at idle, a steady 14 psi

Now the scan tool data:
I get the P0300 code, mutiple misfires
True random misfires on all cylinders, some more than others
Closed Loop
MAF starts at 10 g/s then settles at 8 g/s, original MAF and New MAF (thinking it should be closer to 6 g/s)
MAP 14 In. Hg
Both bank front O2s voltage oscillating between ~100 mv and ~800 mv
Rear O2s voltage at ~800 mv
Short term fuel trims ~0 %
Long term fuel trims ~ -5%

This is where I'm at and looking for suggestions or someone to look at some data, maybe "C5 Daig" can chime in.
Thanks !

 

That's a good start.....do a leak down test as well.

I'm sure you meant 14 In. Hg here.
If you are certain the previous owner did not forget to disclose a cam swap, this needs to be investigated.You should be close to 20 In. Hg at idle with a stock cam if all else is good.

 

Yes, the MAP is in "Inches of Mercury" read from the scan tool .

I'm thinking about the leakdown test since I still have the spark plugs out but that will require buying a new tool ! Might be able to borrow or rent one.

I agree that the old school vacuum test is low. I think a leak down test is needed to rule out bent or burnt valve.

I'll investigate the previous owner cam swap theroy some more but I did not see any evidence that it was swapped. I'm assuming the whole front of the engine and radiator needs to come off to change the cam in the car.

 

If the previous owner had the rockers replace with LS7 rockers it will create a rough idle on cold engine. When they warm up and the other metal parts have expanded ever so slightly, .it is hard to hear or feel. Happened to a friend of mine with a 2001 C5.

 

I was referring to the "old school" gauge that you used to measure the vacuum.

No need to buy, most major auto parts chains rent leak down testers.

Pretty much.

Happy troubleshooting

 

“Used old school vacuum gauge, all looked good except was reading low at idle, a steady 14 psi”

I’d start looking here. If you have a cam worthy of 14” you can’t help but know it. I’d be thinking vacuum leak, and go about eliminating that possibility first. Check all the air tubes and fittings — I’ve had a small gap cause both idle and return to idle problems. If your hands aren’t too big, you could isolate the HVAC at the back of the intake manifold. Other sources are brake booster and PCV. I’m not sure if air injection has a vacuum signal or not.

 

Agree, I know what a healthy cam and/or higher ratio rockers will do.

Yeah, that is the next thing after the leak down test (because I do not want to put the spark plugs back in till I run this test). I'll block off all ports from the manifold and recheck the vacuum with the manual gauge. If the vacuum goes up to 18-20 I'll know its the peripheral vacuum items and add in one at a time to find the culprit. If the vacuum is low with all ports blocked, it must be intake/TB gaskets or some other defect in the intake/TB.

Since I do not know the history of the engine I want to rule out the mechanical stuff before getting into the sensors. The other thing I'm worried about is that there was some tuning done on the ECM/PCM. Why I say this because I can not read the VIN can make from ECM/PCM with my scan tool. Other GM cars I have no problem reading the VIN but they are 2006 and newer.

Is there something special I need to do to read the VIN, I had to do some extra things to get Mode $06 data. I have ODBwiz and OBDLink SX.

 

Ok I ran a couple more tests to see why the vacuum is so low at idle ~14 In Hg.

I did a leak down test and all cylinders were less than 5%, so no valve issues.

I also blocked off all vacuum ports on the throttle body and intake except for the MAP sensor and the port the manual vacuum gauge was connected, again 14 In Hg !

Before I take off the intake and replace the gaskets I'm going to hook up the laptop and check the timing advance. I slightly remember it was higher than I expected it to be it idle.

Any other suggestions ??

 

So if you did not remove the intake, how did you get to the vacuum line for HVAC and A.I.R. ? Where is the gauge connected? Perhaps indicate on the diagram what is blocked and where you are connected....

 

OK, I disconnected the EVAP hose and connected the vacuum gauge to the intake where the EVAP was connected.
I removed the EGR tube from intake and capped intake.
I totallly removed the EGR tubing that went around the back of engine then I was able to pull the HVAC elbow off the intake then cap the intake side.
I removed the tube from the brake booster and capped the tube. I wiggled and moved the tube around while it was running and watching the vacuum gauge just to make sure the booster tube was good. Another thing is when I took the brake booster tube out of the booster it still had vacuum on it after the car off for over 10 minutes. I guess this is what its suppose to do in case you loose engine power you still have power brakes for a while.

I do not think there is a vacuum line for the A.I.R system, or at least on that come off the intak/TB.

 

Since you have a scan tool, use it to check the following:
1. Fuel trims. Neither ST nor LT trims should be high.
2. O2 sensors. Look for a lazy O2 sensor on one bank
3. Check for exhaust leaks at the head to exh manifold. If you hear a ticking sound it ay be an exhaust. leak. This would also throw P0171, 0174 lean codes, but sometimes not. fuel trims will also be up.
4. what is your idle RPM? if not in spec, then maybe a MAF leak.
5. Don’t forget there are several other sensors that will give the PCM bad data. The MAF sensor is one, and a K&N filter with oil is a common culprit. Also look at IAT, ECT and ambient temps to see if all are reading ok. probably good to take the TB off and clean the IAT sensor with MAF cleaner. Easy does it and no compressed air to dry the cleaner.
5. Any history codes for knock sensors. They can get erratic and the PCM will pull timing.
6. plugs, wires. Change them both on all cylinders. But usually you will get cylinder specific misfires, and since you are not reporting that, kind of rules out plugs/wires.
7. Chuck your Mass air flow at idle, should be around 5-6 grams/sec at idle. They get old and the elecrronics start to crap out and will report higher than actual mass flow in to the engine. Sometimes previous owners take out the screen thinking this is good. While the C5Z06 throttle body did not have the screen the base LS1 did. The TBs are different part numbers.
8. If your fuel trims are high, suspect intake manifold gaskets. That will be un-metered air into the engine, and the O2 sensors will report this to the PCM which in turn will up fuel trims.

 

After the expirement of blocking off all vacuum ports then running the engine with the manual guage with no luck, vacuum still at ~14 In Hg. I took the intake off and replaced gaskets and tried again, same results, 14 In Hg !

I guess the throtle body gasket is next, try to pick one up tomorrow.

Anymore suggestions are apprciated !

BTW lucky131969 I'm about 250 miles due west on 30 of you !

 

I re-read your post. Getting old and forgetful.

Where did you get a replacement throttle body?. The after markets often are junk in the box, and unless GM did a huge turnaround, the TB’s aren’t available as parts at the dealer. I had a huge problem sourcing a TB for my C5Z finally ended up with a good used one from Vette Pros here in town. I also had to replace the connector and wires. The wind makes the harness flop, and the wires break near the connector. There are lots of aftermarket connector with pigtails out there.

Trouble shooting TB issues can be frustrating,. Since your O2 sensors are oscillating properly, you should see ST trims change ever so slightly. In your case, you report steady 5% LT trims. You won’t see the LT fluctuate slightly, only ST.

With your low MAP, check your crankcase vent circuit. When you are looking at fuel trims only look at those PId’s and no others. Most scan tools slow down the sample rate when you select a lot of PID’s

The earlies had a funky PCV routing around the back of the engine and there are a couple of elbows on the big tube that routes to the Drvr side valve cover and cracks with age. I recall that can cause a steady vacuum leak and burned oil small from a faulty PCV.
I haven’t had a pre 2001 C5 in a long time, but my old Y2K C5FRC had the wire go bad on the IAT harness at the intake plenum. That is a common problem with the 97-00 C5’s. The IAT was incorporated into the TB for later C5’s IIRC.

 

No EGR on a C5, so I'm thinking you mean PCV. The vacuum line that provides vacuum for the HVAC also provides vacuum for the A.I.R. soldenoid valve. So if you blocked the manifold nipple, you got the HVAC, A.I.R. and associated tubing out of the equation.

 

I reckon that would put you close to Cedar Rapids.

 

Yeah, I meant PVC.

And yess, close to Cedar Rapids.

 

Any other suggestions ?

I reached out to the previous owner and he said that he did nothing to the car except oil changes and driving it. He is going to check with his previous owner.

I know someone replaced the rockers with Harland Sharp ones, not sure if they are 1,7 or 1.8 ratio ??

The other unknowns are if the cam has replaced or if PCM was messed with.

After trying the throttle body gasket I guess I'll remove a valve cover and measure the lift and rocker ratio and compare it to stock specs. If is stock, probally no one messed with the PCM. If its not stock then I probally need a tune, maube ??

 

OK, I got to drinken and thinking. Not sure if this would be true but, if a larger cam was installed wouldn't the MAF report a little more air coming into the engine at idle for the same RPM and also have low vacuum ??

The fuel trims seem to be ok with my limited knowledge and it does seen to run good above 2000 rpm. Just the idle is rough (expected with a larger cam, at least in old school cars).

Any thoughts ?

 

Replacing a cam usually requires removing the heads. The lifters can interfere with pulling a cam if they are still in the bore.

Besides, a cam by itself is not a great power booster, Most folks with an LS1 engine replace the heads with 243 heads, and an LS6 intake. Also, valve spring change is a must. Are your valve springs painted? At one time theLS6 valve springs were yellow, IIRC. Original LS1 springs had no paint., at least they did not on my 00FRC. Also if you have a dual spring setup that would also be a clue the cam was changed.

I think I have some old OEM rockers to compare I can mail you if you need them. They are in my shop attic, and I may have pressed out the bearings, not sure. just send me a pm and we can discuss.

 

No need to shotgun parts. Carb or MAF cleaner sprayed on gasket sealing areas will reveal a leak as well. I'm sure you have already inspected this, but it is not uncommon to have a crack on the rear of the manifold where the MAP/vacuum barbs reside. I recall the first small blower cams would sit right at 14 in Hg. No other ideas.