hi everyone,

I'll give a little history it will be as brief as possible. Bought a low mileage 91 Z28 w/305 5sp this is where the problems began,

picked it up drove it home approx 1400 miles. had problems but they seemed to clear up and she ran good the last 500 miles. Got her home started tuning her up. wires and cap rotor oil change, plugs. she ran good. I didn't drive her for a few weeks and one day I fire her up, she ran good all day. The next day is when the problems began. I drove 8 miles and she over heated. I let her cool down drove her home and went to work on her, I found a bad EGR valve so I replaced it. decided that I would replace the following parts while there.

1 changed t-stat to a GM 170 deg
2 changed fan switch to JET performance (fans come on now at 195)
3 changed temp sender
4 changed temp sensor
5 changed IAC
6 Changed to hi-flow converters (dual) mine had dual to start with

I went to start the car and she ran like crap.... check and checked and finally after so much frustration I sent her to the shop after 2 weeks she was back on the road running great they had replaced the whole distributor, ECM and found the fuel pump had low pressure so it was replaced.

I was going to used the car for a trip so I went for a long drive and during the drive which ended up being about 275 miles, the SES light get coming off and on and the car would buck badly. I was only a few times then it got more frequent. after watching this happen I noticed also that the speedo would jump at the same time. so I decided I would change the VSS but at last on my way back home the alternator took a dump and she had to be towed.

The next day I ordered a VSS, and replaced the alternator also the water pump because the weep hole was leaking and seeing how I was going to drive the car to Calif. I didn't want any problems. I also at the same time changed the Coil to a MSD one. I fired her up and she ran great. I decided to taker her for a ride so i ran up and down the freeway for approx 100 miles and she was purring like a kitten. The VSS was due in a couple of days so I decided just to go ahead and replace it also when it came in even though the bucking had stopped and the speedo seemed to be fine. (insurance).


So now we get to the rest of the story. i left for Calif. under protest from family and friends to just drive my truck and not the camaro. I should of I guess but I didn't. so I leave and she's running great. i got about 300 miles and cruising about 50 miles out of Vegas and with cruise control on she bucks again only once enough to kick the cruise off. so I just kept going and "BAM" she starts this bucking crap again. short story turned around and started to head back speedo going off and on SES light off and on temp coming up and fuel mileage going to sh*t. Made it to town and finally the SES light stayed lit. I thought cool now I have a code.

well took her to a shop and told them what was going on and the history They got me right in and determined the following. The code was a 42 they said probably the Knock sensor. but no code 43..

The speedo had quit working and they looked at the tail-shaft and said the bushing was worn out so maybe the speedo gear on the output shaft was jumping around, and not making contact with the gear on the VSS, also the tail-shaft was leaking. OK I
can buy That
well I'm still 200 miles from home and they can't fix it but it's running so I head home the SES light is on , no speedo gas is getting used up fast running cool enough. I stop for gas I'm about 100 miles or so from home. I get back on the freeway she bucks a couple of times and sets the SES light but the speedo is working again and continued to work all the way home. I parked the car for the night and started researching I haven't found enough info yet but will continue to search while awaiting comments. anyway back to the end of the story. I started her up this afternoon after checking the codes and I got a 42, I let her idle I checked all connections associated with the EST. but the light wouldn't come back on. So I went for a drive I ran down the freeway awhile turned around came back down the freeway NO light and speedo working fine. no abnormal temp changes, ran her all the way up to over 100 mph. no failure.. So My question(s) are

1 could a failing knock sensor set a code 42 without setting a code 43?

2 Obviously this is an intermittent problem that rears its ugly head after so many miles or after something loosens up etc.. has anyone ran into this before?

3 Could the speedo gear jumping around on the VSS and making it not send a signal cause a Code 42 by any chance. I never got a code for the VSS not working.

When the car runs good it gets great gas mileage I was pulling 23.5 mpg before it started acting up...

any advice including the proverbial get a gun would be great ly appreciated. I have included a pic of the car....

Thanks BTW great forum lots of info... stuff like brakes which mine needs addressing and the suspension stuff both of which will be winter projects after I get this annoying problem figured out lol

 

Try the ignition module. Cheap enough, and if it isn't the problem, it'll fail soon anyway, hehe. Good insurance, at least. Make sure the connections there are good'n clean.

If I were in Vegas, i'd put money on it being the module, or something darn close/connected to it. Lot's of money.

 

Although most aftermarket PROMs (off the shelf) are generally altered little more than for fan temperatures, there may have been some other changes, intentional or not.

For the diagnosis, I'd suggest going back to the more reliable factory PROM, with its more reliable backup programming, and start the diagnosis there. You may also want to research programming your own PROM, which will eliminate any rogue factors from the aftermarkets.

As for the error code, once set, you are reverting to the aforementioned backup modes, which are going to alter timing and fuel mixture. The fat fuel mix and nearly flat timing "curve" are going to cost mileage, power, and probably cause heating. I'd suggest also checking the distributor wiring and ECM harness connectors just to be certain. More times than not, an ECM is declared "dead" or "failed" when the real problem is connections - either internally or externally.

 

check your ground wires

 

thanks for the advice,

I have checked all grounds except under the dash which will happen this week along with the ECM connections, The ECM and PROM are factory new delco, the distributor however isn't and I think I might spend the money and get an MSD unit.

I drove the car again today to church then around town to have lunch, I would have driven it some more but the rain was coming and the car hasn't seen rain or snow since 1999. so I parked it and went to my shop to work on my Motorhome for the afternoon.

I'm going to rebuild the tailshaft housing and get that squared away, I'll also replace the Knock sensor just because! Then I'm going to take her on another long run. It seems that what ever it is has to do with something possibly getting hot and loosing connection and when it cools down it works ok so this of course could be anything from a simple connection to a failing electronic component. ie resistor, diode, ic chip even a simple solder joint.

Problems like this are just a pain as everyone knows because even though the part will eventually fail it might take a lot of test drives and putting up with conditions like the other day before it does.

Thanks again for the advice.

 

Check the injectors with an ohm meter to be sure they are not crapping out on you. Should be around 14-16 ohms each. (not sure on the lower limit of acceptability) They may be heating up and then causing too much current draw from the ECM. Strange things can happen if thats the case.
Coil and or the module could also be giving you troubles after getting hot.