I have a 95 Z28 which has developed a stumble off idle in the last 2 weeks. It had new plugs 1000 miles ago (Bosch platinuims) I checked these and apart from the fact they are a bit sooty ( running rich) they look like new.

Car has done 73K so I changed the spark plugs wires. stumble still there but when I get it to rev it feels more powerful than ever with the new plug wires. My car is an auto and it gets stuck in the stumble zone because it occurs at light throttle.

I have done a few mods airfoil, relocate IAT, descreened MAF, free ram air mod and 160 stat. The car has no cat ( removed by previous owner) These were all done Nov03 and the car has run fine until now. It makes a clanky rattlely sort of noise when it stumbles. If you put it in neutral it stumbles if you stab the throttle but then picks up and revs clean thru to the red line.

What should I check next. I am in the UK so there is not much of a knowledge base over here to work with!

 

I am going through the exact symptoms right now. I downloaded DataMaster and found that I was getting a lean condition causing the PCM to compensate when it went into closed loop. The problem happened at or off idle while the car was still in open loop mode and not compensating.

With Shoebox's help, I started looking for causes of running lean. I was hearing a ticking sound from the back of the block and I found the exhaust manifold bolts were loose causing an exhaust leak.

I also found arcing from the plugs and wires. I am in the process of replacing all of that right now. Should be done by Saturday. You may have a similar problem so check for these things.

 

I had a new ILE elbow last Nov as mine was split underneath I saw this when I fitted the airfoil. This came with an IAT sensor already fitted so I removed my old one and relocated it the other side of the MAF sensor.( I have tried plugging back into the old position IAT sensor) I had the ILE elbow off Sunday When I replaced the coil wire and I made sure it was correctly fitted when I replaced it. I have not checked my exhaust manifold bolts though, I will have a look at them tommorrow.

 

I couldn't feel the leak but I could hear it. I saw some white residue on the manifold which indicated a leak. Good luck!

 

all i can say is check your os sensors putting it on a scanner that can read the car while it is running rpms gas and air mixture ect ect check to see if the os sensors are moving up and down in the number scale...... ,,,the problem wont trip a code on the tech two.......hope that helps

 

Thanks SweetZrag I checked the exhaust manifold bolts today and I found 5 on them were slightly loose so I did them up. My stumble is still there but not so bad. The stumble does not notice so much when the car is cold but when it warms up I can't kick the car down it gets stuck at 1000 revs or so and won't pick up. if I change it down manually the revs rise and it pulls like hell.

when It changes from first to second now it snaps your neck and squirms from the back so the exhaust manifolds must have been leaking a bit for me to notice such an improvement in power after tightening them. I am deffinately moving in the right direction.

Quickslvr2 do you mean like an exhaust gas analiser like they use when they do the yearly MOT test to check your emmisions? I do have a buddy how runs an MOT garage so I could get the emmions checked if that's what you mean? What do you mean by tripping a code on tesh 2 can you explain?

Do I need to get the car checked to see if there are any fault codes, I do have a good laptop by no connection or software!

You have to remember I am a novice mechanic when you start talking about scanners and Datamasters it goes over my head a bit. I am in the UK so I don't have access to some of the stuff you guys have over there.

Thanks thou guys things are deffinately going it the right direction.

 

Read this document by Injuneer. He is the moderator of this board and it will help you understand how to use DataMaster.

http://members.aol.com/InjuneerZZ/ScanMast.htm

Also, download DataMaster from this site and buy the interface cable. You will learn a lot from this program. The program is free toevaluate and you have to buy it after 20 uses.

http://www.ttspowersystems.com/diagnostic.htm

It will help you isolate a lot of other problems you would otherwise go nuts over. It will also display any SES trouble codes that might be present.

Good luck!

 

I have had a look at the links you gave me (Thanks SweetZrag) and I have now got some software to read data off the car, now all I need is a cable to connect my laptop to the Z28. I think I have found a guy in England that makes cables on one off Brent franker's link pages.

http://www.bfranker.badz28.com/fbody/links.htm

I found this guy from this link http://www.akmcables.com/dss.htm

UK based http://www.andywhittaker.com/ecu/ecu_software.htm
(He is on vacation until the end of March so it might be a while)

Thanks Guys I will let you know how I get on.

Andy
http://www.cardomain.com/memberpage/441297

 

Any time.

 

With the interface cable that Andrew makes, and a laptop, the following software is great!!:
Tunercat:~$100.00 (you can download the software from the website, then just pay for the license code) http://www.tunercat.com/

Freescan: Freeware. Great for datalogging and checking and resetting trouble codes:
Download here: http://www.andywhittaker.com/ecu/ecu_software.htm

Tim

BTW, I'm struggling with the same symptoms with my 94 LT1 in my Datsun 240Z

 

I have some new parts on order from the USA MSD coil and a harnees so I can use my MSD 6 AL ingnition and a cable to plug my laptop into the car to check for fault codes

If I had fault codes present would my SES light be on? because at the moment it is not.

I went for a long drive today and the car ran like a pig
It was bucking, pinning, misfiring at high revs where as before it only stumbled if you stabbed the throttle
It was trying to cut out the revs were dropping to 500 in gear at the traffic lights so I would push it into neutral to take the load off

You could feel the misfiring thru the body work it was so bad

Does this point more towards the optispark?

Andy 1995 Z28 Cammaro

http://www.cardomain.com/memberpage/441297
http://www.cardomain.com/memberpage/537476

 

I have changed every part of my ignition system (opti, plugs, wires, coil, coil module, O2 sensors) and I still have the same symptoms (off line low rpm stumble, miss, buck). I have checked all my sensors. I am now convinced that one or more of my injectors are faulty.
BTW, my old opti was in very bad shape, but obviously wasn't the problem.

Once I change the injectors, I will post back. The car is in the bodyshop right now (rolled back into the Z28 while I was out). I should have it back next weekend.

Tim

 

We checked my injectors with a light that you can plug in were the sensors go into the loom to see if they were pulsing at first 3 of them were playing up! not pulsing but then we checked them again they were ok! so we checked spark on those 3 and 2 were ok number 2 plug was such a pita to get to we didn't test it

Andy
http://www.cardomain.com/memberpage/441297
http://www.cardomain.com/memberpage/537476

 

Update on my engine stumble things got so bad I had to park the car up.
It was backfiring coughing and spluttering
Anyway I ordered a new optispark from jason cromer ( it takes a while for stuff to arrive as I am in the UK)

I put on the new optispark and new coil and the car runs fine stumble is gone.

I connected my MSD 6AL ignition and that smoothed things out

I have got my cable now and I connected to the laptop and used freescan

I have a fault code 48 mass air flow sensor failiure.

Does this mean my Maf sensor is knackered and I need to buy a new one.

It's odd because the car is running fine. the only thing that is wrong is the spark plugs are a bit sooty saying things are running rich but it has been like that ever since I bought it last October.

If my maf sensor is broke why is the car still running is there some default setting my PCM is using

Andy
http://www.cardomain.com/memberpage/441297
http://www.cardomain.com/memberpage/537476

 

Andy, the PCM will default to speed density upon MAF failure.