Nice catch Stan about draining the TC too, forgot that one step and given the symptoms, it would be a crucial step to do to remove as much of the old fluid from the transmission and TC.

Lon, if your weather is close to what we have today, it should have been a near perfect morning drive in your 240Z! Had a modified 280Z back in the day and loved the car, wish I never sold it.

As far as the correlation between the C7 Corvette and the 928 TT system, they are very different.

What you describe during your driving and application of throttle inducing drive line shudder sounds very similar to the kind of issues owners are having with their GM models which have with the A8 transmission in them, not only the Corvette.

Good luck with the further diagnosis.

 

Stupid stuff --

Check that the drive plate at the front of the torque tube isn't damaged. That would be cracked or broken. This is the front drive flange that bolts to the ring gear/flex plate. Verify that all is tight.

Check that the new bolts at the rear drive plate are still tight. If they are any less than tight, plan on replacing them all, and use the proper bolts.

Double-check the clamps and clamp bolts on the driveshaft.

 

Hi all and Happy New Year. Lets all have a great problem free 2020 with our 928's. Anyway, wanted to update and thank everyone for you advise and suggestions on this issue. OK, now for the update:
If you read through the post you know that I had made several repairs and/or corrective actions to try and alleviate this shudder as suggested by many of you. Half shafts, fluid changes, flex plate, TT inspection, etc. Nothing seemed to correct it until my mechanic Rolly Alberto returned from the Philippines for two weeks. As many of you know from some of my post, Rolly is probably one of the best "old school" mechanics and is the only person I allow to work on the 928 or my 240z. He was a very famous quarter mile racer in the Philippines back in the 80's and 90's and was know to build and modify some of the best 4/6 cylinder racing engines at the time. He has a very strong following still and goes back there from time to time to service some of his "money no object" clients.
I had told him of my issue and he came over. We started the car got in and while driving he said that it was misfiring and probably one of the injectors was bad. He said that it could be the problem. We returned to my house and he pulled each injector clip listening for any change in the rhythm of the motor. He identified the number 8 injector as being bad and suggested I replace all of them. I ordered a set and when they arrived had an issue with the bottom manifold rubber o ring being too small in diameter. Ordered the correct size from roger and installed them yesterday. Finished last night. Car started up after a few turns and no leaks were noted. ( I also replaced the fuel filter as well). The car was surging slightly from 800 -1200 rpms. I decided to wait until this morning to take her out.
Got up this a.m. went out and she started up with no surging noted. Went out and drove for about 45 minuets alternating speeds. NO VIBRATION AT ANY SPEED OR RPM!! So it was just a bad injector for future reference. HOWEVER, I did notice some hesitation at low speed and when I got home the surging was present at idle, both in park and drive. The injectors are 4 hole vs. the original 2 hole and are rated up to 24lbs. Other than that, they appear identical to the old one's. ( they do have a plastic vs metal body of my old one's)
She runs very smoothly at high speed (90-105 for about 3 min.) 75 -85 extended period of time. I checked vacuum hoses and other electrical connections. Air cleaner was serviced and I also sprayed MAF cleaner into the MAF sensor housing while I had the breather and MAF housing out. Could that be the culprit?

Thanks and I look forward to any suggestions.
Lon

I found this fuel line on the the fuel regulator. It was replaced too. All my other lines are Greg Brown upgrades and/or replaced previously.

 

Your new #24 injectors are over-fueling unless you have a) a #24 chip set from someone with a Sharktuner or b) you have PEMs and a Shartuner and have changed the injector type from #19 to #24.

Your sig shows “ECU Upgrade” What’s that?

 

Hi Lon,

Congratulations on finding the root cause of the vibrations! From memory this cause has come up before as a 928 vibration generator and apologies for not thinking of it before. Thought it had already been covered by someone else.

Good luck with correcting the surging, seems Dave is pointing you in the right direction.

Have a great New Year!

 

Good morning Worf928, I will double check with the manufacturer this afternoon, but I thought they said up to 24lbs. You are probably right though, they may be 24lb injectors which would account for the strong exhaust and fuel smell at idle. The upgraded ECU was I purchased from a Rennlist member who specialized in providing a much improved ECU. My original went out 4 years ago. No advanced chip or tuning just industrial grade components.

 

Thanks C,
At least everything I did was a benefit and just made my car better.

 

That’s not the way these injectors work. They are rated for X pounds/hour at Y PSI system pressure. The amount of fuel delivery is a function of how long they are open. Since your LH thinks it had 19# injectors you are getting roughly 125% of the required fuel under most conditions. At idle, the O2/ISV control loop is trying to correct for this over-fueling, but runs out of range and thus the hunting. At speed, the adaptive 02 control loop can compensate for ~80% of this over-fueling which is why it runs ok.

You have THE WRONG INJECTORS in your car and that is the first place to look for your current issues.

John Speake of JDS Porsche?
Rich Andrade of Electronik Repair?

 

Hi Worf, John Speak did the ECU for me. I have attached the flow info from OSIDETGER, the seller that I purchased my injectors from. Sorry about the late response I just got back in town. Marine928

Flow size: 220cc/min/43.5psi
Flow size: 248cc/min/55psi
Flow size: 255cc/min/58psi
Total system: 1760cc/min/43.5psi
Rail size 14mm, Bosch, EV1 Resistance: 14.5
Dead time: 0.315ms@13.8v/43.5psi

 

Higher rate than 19# injectors. Like a 20.5# injector.

Do they look like this? Or not?

 

 

Injectors are typically spec'ed at 3 bar (43.5psi), regardless of what pressure they are actually used at. So our stock 19# injectors flow 19#/hr at 3 bar, and a bit more at the 3.8 bar fuel pressure that the S4+ runs at. Your new injectors are spec'ed at 220cc/min @ 3 bar, which is 24#, which is 25% added fuel, i.e. a richer mixture.

See Worf's post, above. This is simply more detail, not a different recommendation.

The other injector spec that is important is latency, also called opening-time or dead-time. The dead-time spec for these injectors is 0.315ms, which is quite fast-- much quicker than the 0.94ms for stock injectors, in fact implausibly quick-- the fastest I've seen for these types of injectors is 0.71 ms. But if that dead-time spec is correct, what it means is 0.6ms more fuel is being squirted per pulse than a stock injector. A typical pulse-width at idle or light-throttle cruising is around 2.5ms total, so 1.6ms of actual fuel and 0.9 ms of latency. The same 2.5ms pulse with these injectors would be 2.2ms of fuel, which is another 35% added fuel. That's a boatload, in technical terms.

The three injectors at the bottom of your pic are marked "0 280 150 730" which is the Bosch part# for our stock injectors. The other eight appear unmarked, which means they are not Bosch. Flow-rate is easy to measure, and that spec for these new injectors is believable, so I would tend to believe it. Latency/dead-time is hard to measure, this spec seems unrealistic, and the manufacturer is a mystery so I tend to not believe it. I think the kind thing to say is that the dead-time spec is "unknown".

The LH uses the factory O2-sensor to adapt the fuel injection for actual air/fuel ratio, up to +/- 20%. This is a long-term "adaptation" to compensate for variation in parts and MAF aging. This adaptation correction is stored in the LH (as long as the battery stays connected) and always active. In addition there is another +/- 20% "adjustment" available on top of that, this is a real-time adjustment that is not stored, and only active at idle and cruise-- not high load/high RPM.

So what this means is that the LH can almost compensate for the 24# injectors, but then you are flat out of correction range-- before compensating for any other differences or MAF aging. And whatever the latency actually is, it almost certainly does not match the stock Bosch injectors-- even Bosch injectors of different flow rates vary over a 0.5ms range. So very likely you are beyond what the LH can compensate. Yes, it will likely run OK, but it won't run like it is supposed to.

You cannot substitute different injectors and expect the engine to run as it was meant to.

The best thing to do is stick with stock injectors, unless you really want to dive into tuning. Send your stock injectors out for cleaning and flow-testing. If you need extras, there are plenty around. There is a slight performance advantage to disc-type 4-hole injectors, but they don't make 19# versions and any advantage is nowhere near enough to compensate for the wrong specs.

I hope this helps. If you want to dive into the deep end and learn about tuning, I can help.

Cheers,

 

You have to be very careful with injectors as available performance data suggests flows are all over the place despite nominal ratings. Based on performance data in my possession and a quick calculation, the flow rate advised is more in keeping with a 22lb injector rating give or take a little.

Take a close look at the injector and you should be able to see a stamping on the injector plastic upper housing. The stock design 1 Bosch injector has part number 0280-150-730 stamped on it. Take a look at the injectors in your pack and see if you can find a similar part number structure. If you can advise that I may be able to tell you a bit more about the injectors in your possession. I suspect they are a Design 2 4 hole injector similar to those on my motor but those are nominal 30lb injectors. Sharktuner can cope with such no problem but a stock ECU will have problems trying to control AFR on cruise mode and will run pig rich [close to AFR 10] on full throttle..

 

@jcorenman Thanks for the exposition on opening time. That was going to be my next post. You saved me the trouble.

Horse has been led to water.

 

One more thing, those refurbished not-19# injectors need have their exposed bare steel bodies painted else they will rust in short order.