My ’87 951 is experiencing backfiring in the intake and will not idle.

Recent major work:
- Head rebuilt and installed w/new head gasket
- Belts replaced tensioned
- Water pump, thermostat and hoses replaced
- Front seals done
- Turbo rebuilt
- Cat-bypass installed
- New plug wires
- Vitesse MAF installed
- 55# injectors w/ballast resistors installed
- Rebuilt ISV installed
- Venturi delete completed

Car ran fine for about 300 miles. Lumpy idle when warm was only issue.

~ 2 weeks after work done the rebuilt turbo seals failed, turbo blades struck housing (upon inspection)
Car still running but no boost, drove the ~1/2 mile home, still idling ok, lots of smoke out the tailpipe

All work performed by me:

- Removed fuel rail/injectors
- Removed intake
- Drained coolant
- Replaced thermostat (thermostat installed was wrong temp)
- Replaced DME Temp sensor (attempting to solve warm idle problem)
- Removed MC heat shield
- Removed MAF
- Removed turbo

Upon receipt of rebuilt turbo from vendor:
- Replaced turbo
- Replaced MC heat shield
- Filled coolant
- Replaced intake
- Replaced fuel rail/injectors
- Replaced MAF

Current issue(s):
- Will start but won’t idle
- On startup rpm’s rise to ~2500 RPM then drop immediately
- Can keep running with foot on accelerator but just barely
- Backfiring in intake
- Backfiring has popped silicone coupler on throttle body off twice
- Fuel rail popped off of #4 cylinder injector (bolts holding the rail on properly)

Trouble shooting so far:

- checked plug sequence on distributor
- Confirmed distributor mounted properly to cam assembly
- Confirmed plug wires seated on plugs
- All vacuum hoses recently replaced. Double checked all connections.
- Inspected intake gaskets. #1 gasket installed backwards. Removed and re-installed properly. Torqued intake bolts to 15ft/lbs. Thought sure I'd found it!
- Discovered vacuum line to BOV pinched – fixed

Next steps
- Check timing to see if belt skipped teeth
- Check connector on DME Temp sensor

Any thoughts and suggestions would be greatly appreciated, I'm at my wit's end. I really think it's something I did on reassembly, as I said the car ran fine after all the work for 2 weeks, and even ran and idled after the turbo failure.

Thanks.

 

Check the rotor set screw. Similar happened to me when the set screw fell out.

 

Thanks RKD, I forgot to mention in my 'novel' that I did pull the distributor cap off and inspected the rotor. Set screw was in place and rotor would not turn.

 

Flywheel sensor?

 

Maybe, but it seems odd that it was fine before I put the new turbo in and I didn't touch it during the teardown/reassembly.

 

I would check that the cam belt is still timed properly -- marks line up on cam gear and flywheel at same time?

Big vacuum leak?

I'd also check fuel pressure (and make sure your fpr is not leaking gas into the vacuum line).

Compression test (in case turbo grenade cause damage somewhere).

TPS working and adjusted?

 

On first start up after installing the Vitesse ST2 kit (turbo, MAF, piggyback etc) I had backfiring in the intake. John said this was due to a lean condition.
My WBO2 showed 19:1.(duh!)
When I got the afr right the backfiring stopped.

 

Intake backfiring is pointing to a lean mixture. I wound start the process of elimination with the most obvious and easiest to get to tests first. Compression test. fuel pressure, injectors, timing.

 

Wow - you got excellent advice on this thread.

 

Intake popping is a lean mixture. Few pointers (in addition to what others suggested already):

- Have you modified any of the switches on the chip/board?
- Have you checked FQS settings. Compare to document you received with chip.
- You should be running 3bar Fuel Pressure
- Make sure the MAF is getting switched 12Vdc. (high probability!)

 

Thanks all, I really appreciate the help. (EDIT) Sorry for the long winded description and thanks for reading my novel, I wanted to document what's happened as thoroughly as possible both to facilitate troubleshooting advise and to document for the next schlub that runs into this

John: Would the car even start w/o 12Vdc to the MAF? I'll double check that this morning, I hope it's something as simple as that! I'm using the test port for power but have never been very happy with the connection I made.

- Will check timing, although it's unlikely since the car was running even after the turbo went
- Will look again for big vacuum leak
- No mods have been made to the switches on the chip/board
- No changes have been made to FQS settings
- 3bar FPR is in place

 

Check the 12V signal to the MAF...

 

Ok John, pulled the connector off the MAF, key on, put probe from meter into the pin with the red wire on the MAF connector, grounded COM connection on meter and saw 11.88 Vdc...unfortunately for me.

I thought sure that was going to be it.

 

Add to your list of items to check (might need a helper)

- When car is running, read voltage on DME pin#7.
- Check the connection to the engine Temp Sensor.
- While car is running, unplug injector connectors one at a time, notice any difference in running. (You should notice a difference, if no change, then investigate further).

 

Quick update:

- Triple double checked the timing today and the mark on the cam sprocket and the OT mark on the flywheel are in perfect alignment.

- Pulled off the connector for the DME temp sensor. I had replaced this sensor after the turbo failed and was suspicious that it might be the cause. Sprayed the connector on the connector on the sensor with contact cleaner and reconnected. RESULT: No change, still intake backfire

- Pulled the plugs. Plugs were black and damp. When the turbo failed the seals failed I had lots of oil blow-by (or is that suck-by on a turbo?). Lots of smoke out the exhaust and of course the intake tract got a nice coating of oil I'm sure. Cleaned the plugs up with carb cleaner and a nylon brush and re-installed. RESULT: Much better! Engine runs and sounds normal above 1500rpm or so. Still idles low/lumpy/badly with some intake backfiring. Only got to let it run for about 5 minutes tonight, will try more tomorrow.

So, my question is, could the oil that's coating the inside of the intake manifold, hard pipes, IC and pistons be contributing to/causing this intake backfire issue?


- Still to try: Voltage reading on DME pin 7 and pulling injectors while running