the flyin' scotsman

My car has now developed an exhaust backfire on cold start up which cures itself once the car warms, time approx 2 mins.

History: car started this cold 'stumbling' this year (after winter storage and a failed fuel pump). The stumbling like the recent backfire disappeared after a few mins of running. Engine starts fine and runs smooth when warm although at a high 1100rpm idle.

The ignition components are stock.... new or have few k miles on them although I do run Nology plug wires, possibly an issue although the cold and warm state is throwing me.

Fuel system is new/rebuilt with new fuel pump and filter, intake refresh with refurb injectors, new fuel lines, AFPR, stock dampers.

Both brains are fitted with JDS PEMs and the LH is a rebuilt JDS unit............the PEMs/car have not been sharktuned.

Car runs excellent when warm with great power.

Once it gets a little darker I start the engine in a dark garage and see if theres any sparking going on but again the stone cold vs warm is baffling.

BTW...............temp II tested good with a DVM...............any other tests it should have?

Lizard928

I would recommend cleaning all the grounds on the engine.
Engine to chassis and above the fuse panel.

the flyin' scotsman

thx Colin.

As you know when we first installed the PEMs the 'stumbling' issue was there and you thought at the time it was an ignition issue. The issue is slowly getting worse with the the recent backfiring and its only when the engine is stone cold.

Its as if one or more plugs are not firing until the engine is slightly warm then its absolutely fine..........the plugs are new, OEM copper and heat range; issue existed with the previous quad 4 plugs.

I'm assuming the high idle is the ISV and/or leaky throttle shaft/flappy brgs and is unrelated to the cold misfire issue.

the flyin' scotsman

Dark garage engine start revealed no issues with the plug wires.

Anyone with temp II issues have similar probs?

I'll clean all grounds tomorrow and report.

jpitman2

Backfires usually mean its lean. Check the temp II sensor leads at the LH brain? And the earths...
jp 83 Euro S AT 52k, CIS...

AO

Malcom-

So it sounds like this is a stock car, with the exception of the PEMs, right?

I noticed with my 42# injectors that the warmup enrichment map was far too rich for my car. I had to basically zero out all the values and even then, it still runs rich. But I never had any back firing issue.

Do you have a WBO2 sensor you can put on it to see if it's rich or lean?

Can you tell which cylinder is not firing from the plugs?

How about cam timimng, cap, rotor?

the flyin' scotsman

thx Andrew,

Car is basically stock with few mods..............x pipe with new o2 sensor was fitted a number of years ago. No WBO at home although I do part own a STII which could be used when its available.

Cam timing was set with Porkens great tool, caps and rotors are a few years old but not many miles (25k kms), wires are same age and mileage.

Its a sensor or switch like issue where either time and/or engine temp resolve the issue from stone cold start ~ 2mins after start. Repeat, the car starts and runs very well once its over this issue.

MAF is a rebuilt JDS unit again a few years old now.

John Speake

It does sound as though it is running weak before the O2 loop activates at about water temp = 60degC.

AO

So what would cause this?

Since the car is running running (ohh crap... I can never remember if it's open loop or closed loop...) before the O2 sensor is correcting the AFR, the only other thing that I can think of that regulates how much fuel is delivered is the MAF. Agree?

Interesting description on Wikipedia about backfires.

John Speake

Well, yes MAF, fuel pressure, temp 2 sensor.

Malcolm, does the car appear to run weak in that first 2 minutes ? i.e. poor throttle response ?

Temp 2 sesnor readings when cold ?

AO

Good point. I should have known this because I just got done helping a local who's car is always lean. Check the Temp II and found it was a bit flaky - wiring was a bit ef'd up. Then we put a fuel pressure gauge on and fuel pressure was only 20psi. 044 pump is on the way.

Lizard928

Malcolm,

Couple more questions for you.

Is your air filter in the right way and when was it last checked/cleaned/replaced?
Is it stock or K&N?

Have you checked the wires on the MAF/temp II to verify that there is no broken/shorted contacts?

Maybe check the Temp II resistances in the LH plug prior to starting, what are those values coupled with engine values?

the flyin' scotsman

John............the car is essentialy undriveable in the first 2 mins from stone cold start.............I have to wait until it clears itself then its absolutely fine.

the flyin' scotsman

Colin.......do you recall which points should be tested on the LH connector for the temp II?

Air filter is new and OEM.

AO

LH and EZK pinout are in the diagnostic thread in my signature.