Need Advice: Miss, low rpm stumbling - Page 2 - ZR-1 Net Registry Forums

tomtom72 · 07-10-2012

Bingo on the vac pump cycling every 15 or so....that is a slight vac leak somewhere. Now discon the hose at the right water outlet tube and cap the fitting and try the key on engine off and see if the pump stops running after she pulls a vacuum. At the end going under the plenum hook up a vac tester and pull a vacuum and see what happens. Any vac leak adds more air/leans out the A/F ratio, so yes that could cause a miss.

The intermittent SES light will have set a DTC if it stayed lit long enough and the DTC will be stored as a History DTC in the CCM.

Call out to the WAZOO, as The Captain has a tech1A and that will be the best choice to see what is going on.

Okay on the filter, and the socks are proly the original ones. IDK what to say about the socks. Mine were clean after 17 yrs, but the original pumps were down on output so I changed them out. The butt-dyno could tell the difference between old OE pumps & new Bosch pumps. Just saying.....

Tom

gunman0 · 07-10-2012

You can see if there is a bad check valve for the secondary system by pulling the vacuum line to the middle of the driver's side of the plenum. Put your finger over it and see if the vac pump stops running every 15 seconds. If so, you can just throw another check valve there as a quick fix until the next time you pull the plenum.

I don't think a secondary vacuum leak would cause the low rpm miss like you describe. Also the fact that it picks up power over 3000 rpm seems to negate that as being a major problem.

scottfab · 07-12-2012

Forget all effort into the secondary vacuum for now. Cycling every 15 min is not perfect but not going to be a big issue yet. When it stays on, then worry about it. Once you've found your miss then you can come back.

Yes do get a scanner. Don' t waste time with any "short two pins" and "watch the led flash" and "count the flashes".

Spend time on the 45 psi fuel issue. It may not be the source of miss but is low and will cause hard starts (long start time) and shows the pumps are tired. I would not settle for less than 50 psi. How many seconds does the engine crank before starting?

Check the codes and go from there. However, checking the MAP hose is easy and has been known to cause issues like you are describing.

EWest · 08-09-2012

I have an update on my woes, and was looking for a few opinions. I finally got frustrated enough, combined with a lack of time to tinker recently, to take my Z to a local Corvette shop who has a guy that is familiar with the lt5 and has a good electrical guy.

At first they found a cracked spark plug at #6, which made me do a celebration dance in anticipation of it being fixed...but not so fast.

She still stumbles and sputters under load, but does not miss at idle.

He replaced the coils, the miss came back and same problem. SO he isolated the problem to cylinders 4 & 7. He thinks it is the DIS, but "thinks" is not enough for me to drop that kind of coin on a new one. He's in the process of checking all of the wiring around the coils, module, and ecm and so far everything has ohmed out ok.

Any thoughts?

scottfab · 08-09-2012

Quote:

Originally Posted by EWest

She still stumbles and sputters under load, but does not miss at idle.

It would be nice to know if this same behavior is exhibited with the power key off. And what does the fuel pressure drop to when under load? I'm concerned that all the effort is going to spark.

Kevin · 08-09-2012

Quote:

Originally Posted by EWest

I have an update on my woes, and was looking for a few opinions. I finally got frustrated enough, combined with a lack of time to tinker recently, to take my Z to a local Corvette shop who has a guy that is familiar with the lt5 and has a good electrical guy.

At first they found a cracked spark plug at #6, which made me do a celebration dance in anticipation of it being fixed...but not so fast.

She still stumbles and sputters under load, but does not miss at idle.

He replaced the coils, the miss came back and same problem. SO he isolated the problem to cylinders 4 & 7. He thinks it is the DIS, but "thinks" is not enough for me to drop that kind of coin on a new one. He's in the process of checking all of the wiring around the coils, module, and ecm and so far everything has ohmed out ok.

Any thoughts?

sounds like he's throwing parts at it. did you check the injectors? fuel pumps?

EWest · 08-09-2012

Quote:

Originally Posted by scottfab

It would be nice to know if this same behavior is exhibited with the power key off. And what does the fuel pressure drop to when under load? I'm concerned that all the effort is going to spark.

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

EWest · 08-09-2012

Quote:

Originally Posted by Kevin

sounds like he's throwing parts at it. did you check the injectors? fuel pumps?

Injectors tested out fine, fuel pumps test ok, but like I said I have not had the chance to test them under load.

XfireZ51 · 08-09-2012

4-7 are on the same coil pack. If this guy knew much about LT-5s he'd know not to suspect the DIS module.

scottfab · 08-09-2012

Quote:

Originally Posted by EWest

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

absolutely. If you get to a low pressure situation one cyl is going to go lean before the rest. The "miss" has to start somewhere.
I assume the wires were checked out? Length on a few are the same so they can be swapped. AND if one plug was cracked I'd not rule out more damage to others. These aren't those funky Bosh plugs are they? I've seen multiple problems with those.

07-10-2012	#11
tomtom72 Join Date: Dec 2004 Location: Jacksonville, FL USA Posts: 4,609	Re: Need Advice: Miss, low rpm stumbling Bingo on the vac pump cycling every 15 or so....that is a slight vac leak somewhere. Now discon the hose at the right water outlet tube and cap the fitting and try the key on engine off and see if the pump stops running after she pulls a vacuum. At the end going under the plenum hook up a vac tester and pull a vacuum and see what happens. Any vac leak adds more air/leans out the A/F ratio, so yes that could cause a miss. The intermittent SES light will have set a DTC if it stayed lit long enough and the DTC will be stored as a History DTC in the CCM. Call out to the WAZOO, as The Captain has a tech1A and that will be the best choice to see what is going on. Okay on the filter, and the socks are proly the original ones. IDK what to say about the socks. Mine were clean after 17 yrs, but the original pumps were down on output so I changed them out. The butt-dyno could tell the difference between old OE pumps & new Bosch pumps. Just saying..... Tom __________________ 1990 ZR-1, Black/grey, #2233, stock. ZR-1 Net Reg Founding Member #316 & NCM member

07-10-2012	#12
gunman0 Join Date: Mar 2012 Location: Balt Co, MD Posts: 41	Re: Need Advice: Miss, low rpm stumbling You can see if there is a bad check valve for the secondary system by pulling the vacuum line to the middle of the driver's side of the plenum. Put your finger over it and see if the vac pump stops running every 15 seconds. If so, you can just throw another check valve there as a quick fix until the next time you pull the plenum. I don't think a secondary vacuum leak would cause the low rpm miss like you describe. Also the fact that it picks up power over 3000 rpm seems to negate that as being a major problem. __________________ 91 Black ZR1 DRM Chip Corsa cat-back

07-12-2012	#13
scottfab Join Date: Jan 2009 Location: Portland Oregon metro area (Washington side) Posts: 3,193	Re: Need Advice: Miss, low rpm stumbling Forget all effort into the secondary vacuum for now. Cycling every 15 min is not perfect but not going to be a big issue yet. When it stays on, then worry about it. Once you've found your miss then you can come back. Yes do get a scanner. Don' t waste time with any "short two pins" and "watch the led flash" and "count the flashes". Spend time on the 45 psi fuel issue. It may not be the source of miss but is low and will cause hard starts (long start time) and shows the pumps are tired. I would not settle for less than 50 psi. How many seconds does the engine crank before starting? Check the codes and go from there. However, checking the MAP hose is easy and has been known to cause issues like you are describing. __________________ Scott Vett owner since 1979._It's about the car and the people

08-09-2012	#14
EWest Join Date: Jun 2012 Location: Wilmington, DE Posts: 35	Re: Need Advice: Miss, low rpm stumbling I have an update on my woes, and was looking for a few opinions. I finally got frustrated enough, combined with a lack of time to tinker recently, to take my Z to a local Corvette shop who has a guy that is familiar with the lt5 and has a good electrical guy. At first they found a cracked spark plug at #6, which made me do a celebration dance in anticipation of it being fixed...but not so fast. She still stumbles and sputters under load, but does not miss at idle. He replaced the coils, the miss came back and same problem. SO he isolated the problem to cylinders 4 & 7. He thinks it is the DIS, but "thinks" is not enough for me to drop that kind of coin on a new one. He's in the process of checking all of the wiring around the coils, module, and ecm and so far everything has ohmed out ok. Any thoughts?

08-09-2012	#19
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,686	Re: Need Advice: Miss, low rpm stumbling 4-7 are on the same coil pack. If this guy knew much about LT-5s he'd know not to suspect the DIS module.