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Old 04-01-2014, 01:01 PM
ngoma ngoma is offline
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Join Date: Nov 2009
Posts: 1,297
Default Hard Starting

Hard starting sticky. Currently a work in progress. Please make your comments and I will include your additions/corrections in the sticky.


Hard starting usually caused by fuel, glow plug, or compression issues.

Compression issues include low starting RPM caused by a weak starter, battery, or starting circuit wiring.

An otherwise healthy IDI engine needs the following ingredients for good starting:
  1. Sufficient starting RPMs
  2. Sufficient fuel and pressure to injectors IP timing needs to be close to spec.
  3. Glow Plug heat

Glow Plug Section

D24 really needs ALL 6 GPs functioning for good starting. Depending on ambient temperature and general engine condition, ONE bad GP can cause a no-start condition. TWO bad GPs can cause no-start even in mild ambient temps.

Quick'n'dirty GP testing:
  1. Visually inspect GP wiring and connections, especially the large diameter wires from battery + terminal to GP relay, and from GP relay to GPs, and GP bussbars. Clean corrosion if present. Disconnect battery first.
  2. Measure voltage at the GPs when the system should be energized. Have a helper turn the key to the on position when the engine is cold, while you measure voltage at the GPs is near 12VDC.
  3. Measure continuity of each GP individually. Remove bussbars and measure continuity between GP screw terminal and hex flats. DVOM should read 0 ohms or close to it. 1 ohm indicates faulty GP. You can definitively better test the GPs by removing them, and energizing them directly off the battery. Use thick wires (I use jumper cables) and watch for the GP tip to glow orange/red within 6 seconds.

More on testing GPs here.

Air in Fuel Section

Hard starting-- air in the fuel system.

Fuel system needs 100% liquid supply. Air mixed in with liquid fuel will cause problems.

Performance problems or hard starting problems can often be traced to air intrusion into the Injection Pump.

We often ask to install a temporary section of clear tubing or hose to the IP return fuel hose. This will allow visual inspection of air bubbles (or not) in the IP return-to-tank fuel line.

We use evidence of air in the IP return to infer air intrusion into the IP, where there should be no air. Air in the IP most often points to one of several common failure modes:
  1. Failing IP mainshaft seal allows air ingress while engine is not running, which subsequently causes hard start condition. Not usually a performance problem once running, as that section sees (minor) fuel pressure when running, not vacuum. It may leak fuel when running. When stopped, gravity wants to pull standing fuel away from the IP back towards the fuel tank. A leaky IP shaft seal will allow air into the IP to displace the fuel draining back to the tank. Restarting the engine is difficult because the IP, now full of air instead of fuel, struggles to send sufficient fuel to the injectors. The longer the car sits, the more air enters the IP, the harder it is to start.
  2. Loose connection or leaky hose in fuel supply run to IP. Or the presence of an extra fuel filter sealing o-ring (the smaller one of the two). Often the smaller of the two filter sealing o-rings is not seen and therefore not removed during a filter replacement. Since a new o-ring is usually supplied on the new filter, the filter gets installed with two o-rings sandwiched together, allowing air intrusion into the IP. This failure mode can cause hard starting, and almost certainly poor performance, especially under WOT (high fuel demand) conditions. More on fuel filter/o-ring here.
  3. Loose connection or leaky hose in fuel return from IP to tank. Problem starting usually only after parked facing uphill. Air can enter via a loose IP fuel out banjo bolt or injector fuel return hose.
  4. (Turbo model only) Leaking IP fuel enrichment pin o-ring in LDA (the "flying saucer on top of the IP) can allow air intrusion. You may also find evidence of fuel leaking out of the atmospheric vent fitting, located just above the IP fuel out banjo bolt.

Test for fuel to injectors by loosening an injector to injector line sealing nut, crank engine with ignition on, and verify small spurts of fuel forced out of the end of the injector line. Just small drips of fuel out the end of the injector line, not foam. Take care to avoid getting fuel on any hoses, it will easily damage them. Do not overtighten the nut when reconnecting. Injector line connector nuts torque spec: 25Nm / 18ft-lbs.

Compression Section

Hard starting-- Compression

IDI Diesel engine needs good compression for starting. Borderline low compression engine may start when cold (because GPs will energize, momentarily helping overcome low compression), and hot (hot engine makes higher compression) but not warm, like say after a 20-30 minute shutdown period. Engine is not hot enough to provide good compression, but not cold enough for GP controller to energize the GPs.

Sometimes you can cheat this condition by forcing to GPs to energize. By disconnecting the GP temp sensor at the rear of the head, you fool the GP controller into thinking it's always max cold out and to energize the GPs for the max time period. The temp sensor sticks out of the rear of the head, near 8 o-clock position along the rear camshaft pulley. Brown wire (if it's original), don't let it get tangled in the IP drive belt. Note this may prematurely wear out the GPs but will buy you some time until you can address the low compression issues.

Low compression is usually caused by worn piston rings/cylinder bores, or valves needing adjustment.

Engine sat for a long time (years):
Piston rings can lose their seal. I had an engine that refused to start. Engine had sat for years, recently installed in car, needed it road-worthy like yesterday. Fuel was spurting from injector lines, compression was known good (when it was last running), GPs tested good, new starter, good battery backed up with charger, etc. Finally we got it started ONLY after squirting 1/4 teaspoon or so of motor oil into each cylinder via the injector holes or GP holes, I forget which, but will never forget how quickly it ROARED to life after getting the oil into the cylinders.

Troubleshooting symptoms

Here are some common symptom modes and what to look at:
  • Car getting progressively harder to start when cold: Look at GPs.
  • Car hard to start after sitting overnight or longer, not necessarily temperature dependent: Look at air in IP (faulty shaft seal or fuel supply hoses). If it doesn't want to start after parked facing uphill, look at loose IP fuel out banjo bolt, or leaking injector return hoses. ALSO: Leaky large o-ring between IP distribution head and IP main body can cause air intrusion/lack of prime = hard starting.
  • Car starts cold or hot but not warm: Look at compression.
1985 744 gle d24t
1985 745 gle d24t

Last edited by ngoma; 08-30-2017 at 09:19 AM. Reason: Added injector line connector nuts torque spec., formatting tweaks
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Old 10-05-2014, 08:19 AM
jpliddy jpliddy is offline
Senior Member
Join Date: Jan 2014
Posts: 327
Default 940 tdi auto 1995

the only hard starting i have had with this car had it 14 years now has been glow plugs need replacing which i have done twice now .
the front 4 are easy to check if 2 are down then thats the problem with ignition off
disconnect live wire/buzz bar that fits to top of glow plugs then use neon tester with crocodile clip connect to live on battery and place neon probe to top of glow plug if neon lights up plug is ok if no light then plug has failed .
i always replace all 6 but numbers 5 and 6 are not easy to replace .im a carpenter and have done it so take your time you will do it
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