Strange Idle relating to average mpg display

[scarylarry]

I think I have a good one here. 87 conv with 73k. Out of the blue my car started idling a little high around 900-1000 rpms. no check engine light. I did some research and checked connections on TPS and IAC and listened for vacuum leaks. I also calibrated TPS to spec and reset IAC. Still have same problem. Now to the strange part! The problem was intermittent and just by luck I noticed when the car started to rev above normal my Average MPG display would change to 12.8, and when the revs would start lowering to normal the display will show 0 and run fine. Oh yea, I did remove power to ECM in an attempt to reset which is why I have a 0 for mpg. Also, when I have a high idle and 12.8 display and rev the motor the car will die like its getting no fuel. I do get a check engine when that happens. I can catch it before it dies by pumping the throttle to keep it running. One more thing! I short A+B on diag port and only get a solid check engine light no flashes at all. Any suggestions?

 

[scarylarry]

update

Can I get a response from someone? To update I did check/clean the grounds on the drivers side block, which included around 5 wires. I suspect it's the ECM. Any help out there? I know I'm new to the forum, however it would be nice to have at least one reply.

 

[JimBobC4]

Larry,

Man .... I'm clueless on this one.

My first suggestion would be to check fuel pressure. The goal is to see what its missing (Fuel air spark) when it dies or tries to die.

Something is different when it goes into its intermittent error mode.
( no kidding right ? )

I suspect a lean condition.


Does the car have any codes stored ?

Maybe someone else will come along that has some input. A LOT of really sharp people hang out on this board. Don't write them off yet !

 

[jester]

I've seen my mpg display go to 12.5 when I have a scan tool connected. With the relationship of the mpg display and idle issue occurring at the same time, I would suspect that the problem is electrical. How to find it is a whole different question. Do you have the factory service manuals?

 

[scarylarry]

Thank you JimBobc4 and jester!

I know it sounds wierd but true. JimBobc4, I have not had a chance to check fuel pressure, but I do hear a ticking sound on the fuel rail driver side when I do the screwdriver to ear test against the rail. ??? I did just recently replace the valve seals with the updated kit from GM to prevent smoke on start up. I drove 500 miles after the repairs with no problems with the idle and MPG display. It just started out of the blue.
I would love to tell you what my codes are but I can't get the pin shorting method to work on my diagnostics port?
Jester, I don't have the official the GM manuals only the Haines variety. I've checked some wires running to ECM with no apparent damage. I did remove the ECM and reseat the chip. I think I'm going to take it to a shop and have it testedd with a scanner. Thanks again

 

[gmjunkie]

I'd suspect the Alternator or the Big Red Wire on the back!! Been there done that!! First thing I'd check is the Big red wire that is on the back of the alternator,the eye connector will corrode where it is crimped to the wire and not get a good connection and you wont be able to see it!! But it will sure cause this same problem!! I made a 1,400 mile round trip service call to Florida over this wire about 4 years ago on a Vette I sold to a Customer about 6 months after he bought it!!

 

[Toms007]

How about grounding issues? Junkie, could bad ground cause his problems?

 

[gmjunkie]

How about grounding issues? Junkie, could bad ground cause his problems?

Yes!! And a bad Diode in the alternator can Too!!

 

[scarylarry]

Thanks for the replies!
Wow I feel special to get a reply from GMjunkie!! I'll investigate the alternator tonight after work if it doesn't rain. Keep you posted.

 

[Paul Higg]

Junk is the BEST!

 

[JimBobC4]

learn something new everyday !

 

[Paul Higg]

What? That Junk is the best? He is one of the best I have ever spoken to and he has helped me out a lot. He is a really good man.

 

[scarylarry]

update

Hey Junkie,I had a chance to check out the red wire on my alternator and it looked clean as a whistle. The rubber boot that covers it was in great shape with no corrosion. I am getting between 14.0-14.2 volts on the dash display if that helps. Are you saying I should try a new alternator as my next step?
Thanks

 

[TedC]

scarylarry, I would focus in on the connections to the ECM, or the ECM itself. Two reasons; solid light when the ECM is in diagnostic mode and the odd MPG readings. The ECM sends MPG serial data to the cluster. When I started getting odd MPG readings, a rebuilt ECM solved that issue. Your voltages with engine running are fine.

 

[scarylarry]

Thanks for the input TedC. Do you have any suggestions on the best place to buy an ECM? I have read some horror stories on ECMs being replaced and the vehicle running worse than before!

 

[TedC]

scarylarry, I purchased mine at AA for around $70. They claim these ECMs are cleaned, rebuilt, and resealed to equal or better than GM specifications. I opened up the unit to see how the internals compared, and indeed they did a much better job sealing up the board components. The GM coating does not age well. Plus, I can exchange it for life at no extra cost.

 

[scarylarry]

Thanks again TedC! I will assume your talking about AutoZone? I tried their web site search and nothing came up for ECM. Did you special order it?
It sucks being afraid to drive your car because it might leave you sitting! I can't wait to get this issue resolved and drive my car with confidence.

For the record, this car has been very reliable and has never left me stranded!!!!

 

[TedC]

scarylarry, I looked up my receipt and it was Advanced Auto not Autozone. Cardone Part Number: 777165. According to AA's web site, this part is valid for 1986 to 1989 vettes. Cost is $78, $95 core charge, and a 1 year relacement. AA did not have the part in store stock, but it arrived from their warehouse the following day. Don't forget that you need to swap the prom chip from your old ECM. This is also a good time to make sure all the pins are straight and clean. Before you start replacing parts like the ECM, can you confirm that when you enable the diagnostic mode (jumper ALDL "A" and "B"), that the SES (service engine soon) light is on steady with no flash code 12? If the answer is yes (SES always on), pull the ECM and try again. If SES is still on, you have a short on the SES ground wire (brown/white). While the ECM is out you can reseat the prom chip and check and clean the pins. Be careful of static.

 

[scarylarry]

Thanks TedC !!! You went above and beyond with all the info you provided! I confirmed the SES light was on solid with no flashes. I am 95% sure it's the ECM. Advance Auto in Red Lion had the Cardone 77-7165 in stock for just what you said, $77.99 with a $60.00 core charge. The weather is not cooperating for me to install ECM right now. I also have a replacement Y-pipe to install . My Flowmaster rusted through at the seams and sounds like crap with the exhaust leak. We are getting rain off and on and I don't have a garage handy. Hope the sun is out tomorrow.

 

[vette079]

My thought too, solid ses light when jumping a and b means no good ecm or short in wiring to ecm or faulty memcal IIRC.