300ZX (Z31) Performance / Technical Discussions related to Turbo charging, Supercharging, Engine, ECU, exhaust, and etc. performance enhancements and Techical related.

Cold start issues and lacks power after turbo swap

Old 09-28-2016, 04:43 AM
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Cold start issues and lacks power after turbo swap

Looking for some insight here guys. I picked up a refinished stock turbo and had it put on the car by a local shop. They put on the turbo, replaced a motor mount, fixed some busted exhaust bolts, and threw on a new O2 sensor. I just got the car back and it barely starts when cold, like its chocking and running way lean. Once it warms it its drives but lacks power, gotta pump the gas pedal to get the RPMs up.

Prior to taking the car in, it ran great besides the turbo not pulling much. I ran the codes and its basically saying TPS, MAF, EGR, CHTS, and Crank Angle Sensor are faulty! I think something is jacked up because these all worked prior to the repair, and I put a tested MAF on and its still throwing that code.

I'm taking the car back to the shop this weekend, but would rather diagnose the problem myself so it doesn't get worse. Let me know if anyone has suggestions on where I should start. Here are some notes:

1. Shop did say the turbo was slightly different than the existing one, had an extra hole they had to plug. Bought the turbo from another AE owner who said it was stock.
2. If I unplug the CHTS during cold start it gets more fuel and revs slightly better, but doesn't help performance issue when warm.
3. Wondering if the O2 sensor could be bad, since its the only new electrical component they threw on. Didn't think O2 would affect cold start though.
4. I did notice on the electric dash, the boost meter shows more on startup than before (lights go halfway up). Want to make sure I avoid too much boost issue, don't need a blown head gasket.

Sorry for long post, at a lose on what's going on and its bumming me out!
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Old 10-03-2016, 11:36 PM
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A few things you can try.

Unplug O2 sensor completely, it is not necessary in normal driving around town and the car should run good without it. A bad o2 sensor can cause it to run somewhat differently than it should, however.

Check ignition timing. This is what it might be... The vg30 will run with the distributor a tooth off, but it will feel slowwww. verify this with a light.

The injector plugs are finicky. Unplug each one and you can use a very small flathead screwdriver to gently bend the prongs inward to make sure they get a good contact.

If that doesn't fix it. Then you should disconnect the battery, unscrew the ECU plug clean it with contact cleaner, then carefully put it back together making sure that it goes on straight and tighten snugly. then press on the plug all around firmly with you thumbs to make sure they are all contacting well. I bet this is the real problem because you have SO MANY codes.

Regarding the boost gauge. You should get an aftermarket boost gauge right away, you won't blow a gasket at idle but if it is doing something like the wastegate not opening, then you could cause some damage.
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Old 10-06-2016, 08:03 PM
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BattleGround, thanks for the info! I haven't had a timing problem in 15 years but after this work, the timing was off 10'. Adjusting the timing fixed the lack of power issue 100%. New turbo is pulling nice!

I still have the cold start stall problem which never existed before this work. I'll run the codes again this weekend and test each sensor it reads is malfunctioning. More to come. Thanks again for the suggestions, got me running again.
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Old 10-07-2016, 10:10 AM
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yeah reset the ECU to wipe the codes and see what resets
diconnecting the O2 puts the ECU in a failsafe setting which can be different than normal FI function

this was a turbo and you replaced it ? or was it a N/A ?


grab a multimeter and logic probe ,wiring diag. , paper clip and a vallium if you have it
pre OBD I's were not fun to chase down IC problems
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Old 10-07-2016, 01:25 PM
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@dpark,
I'm glad the timing was the main issue. that is always a relief! Cold start could be a lot of things, mainly I'd just double check your spark plugs and make sure your IACV is clean and not full of crud. I personally deleted my IACV and i adjust idle with the throttle blade.

Originally Posted by 86tcoupe View Post
yeah reset the ECU to wipe the codes and see what resets
diconnecting the O2 puts the ECU in a failsafe setting which can be different than normal FI function

this was a turbo and you replaced it ? or was it a N/A ?


grab a multimeter and logic probe ,wiring diag. , paper clip and a vallium if you have it
pre OBD I's were not fun to chase down IC problems
you're exactly right about the o2, but it is a useful way to simply check if its just the o2 sensor or if it is something else, and will run usually well, though on the rich side.
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Old 10-13-2016, 06:49 PM
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This car was always a turbo, put a new turbo on because it wasn't pulling hard anymore and I came across a redone stock turbo at a great price.

So car runs great when warm, but rich. Cold start is still very stumbly, and its throwing the following codes:

12 - MAF (new one put on, and it tests fine, revs up high and cleans itself as designed)
32 - Starter Signal
43 - this isn't even in my FSM! Not sure what it means.
13 - CHTS (OHMs in spec when cold, going to check when warm)
11 - Crank Angle Sensor (need to test this one)

I'm going to do more tests on the parts above, but I'm wondering if it could be the ECU itself, as that is a ridiculous amount of codes for a car that runs fine warm. Could the CAS being off from bad timing cause other false codes?

Here she is:
http://deanparke.com/images/300zx/rims/P3060007.jpg
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Old 10-13-2016, 09:45 PM
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did you get a chance to unplug and plug the ECU back in? I was assuming the car was manual, but is it an auto? This many problems points to ECU for sure, although there are 2 plugs that are identical in the engine bay one goes to the O2 and one goes to something else. It is possible to mess them up but i cant remember which ones they are.
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Old 10-14-2016, 08:14 PM
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Yep, its an auto transmission. So I had the 16 and 20 point top connectors out, and they were fine, but I just checked the bottom connector and the lower pins were a bit corroded. It was a bitch to get unplugged. I cleaned it the connectors with Deoxit but no difference. Might try an ECU swap next, let me know if anyone has a line on a 84 Turbo Automatic ECU!
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Old 10-14-2016, 08:49 PM
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If it's running rich even when warm that to me really seems like an O2 issue. Very common for O2 sensors to be pieces of garbage right out of the box. If you kept your old one its worth a shot to swap it back on. you said yourself it was the only electrical piece that was changed. The CHTS threw a code. but you checked it cold and it was within spec.
You also have MAF, CAS and starter signal code. Which leads me to think, your wiring harness needs carefully checked. ALL these problems, only after being worked on. all the wires run in the same loom. Old brittle wires + a mechanic shop racing to get the job done = harness issues.
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Old 10-16-2016, 08:25 PM
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I hear you, going to start checking harness wire by wire. Only time I've taken it to a shop in the last 15 years of working on it. The fact that its throwing code 43 which seems to be only for California missions models points to a ECU or harness issue. If anyone has a line on an 84 AE Federal Turbo ECU for Auto trans, let me know.
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