civicservice

1999 Civic DX w/ MT. Bone stock. 240K miles.

Over the last year an intermittent hard start issue became more frequent. When the car was cold, it would start right up. But after the engine warmed up, sometimes the engine would not catch on the first turn of the key. Sometimes cranking again would start the car, but sometimes it would take 2-4 tries. Occasionally the car would sputter when started and stop unless throttle applied. The intermittent nature of this problem made it extremely difficult to diagnose, but tried anyway.

Many forum posts recommended re-soldering the contacts on the PGM-FI main relay. They apparently crack over time and heat can cause them to become disconnected. Examined the main relay. Some solder connections looked like they might be pitted and/or cracked - but just barely. Considered hitting relay with a heat gun to simulate hot condition, but decided against it. Re-soldered the connectors that might have been damaged and replaced unit. After a few start/shutdown cycles, the hot hard start problem was back.

Coolant temp sensor (ECT sensor) was another possible cause based on forum posts. The thinking is that when disconnected, the ECT reports a sub-zero temperature to the PCM. The PCM responds by flooding the cylinders with gas (need to start in cold temps), and the engine floods. Tested ECT with multi-meter. Potential and resistance were both within spec at PCM-reported temps. Jiggled ECT sensor wires but only smooth operation.

On some cars, dirty/damaged MAF sensor sometimes causes symptoms like what I saw. However, this vintage of Civic has no MAF. Instead, the PCM uses the known volume of the cylinders together with MAP reading and a lookup table to meter air. Nifty, but still didn't solve my problem.

One day I got lucky and the car developed a cranking no-start after it had been driven.

Good spark. Injectors pulsing. Attempted to read fuel pressure, but many problems with rented gauge. Qualitatively plenty of fuel based on visible pressure release after cracking open banjo bolt. In an attempt to double check, took off air intake and added propane into throttle body. Still cranking no start. Good compression.

Fuel and spark seemed ok, so decided to look at air delivery. Discovered that car started if throttle pressed during start. However, car died immediately if throttle released. Car would also start if brake booster hose pulled (connects after throttle body). This pointed in the direction of a stuck idle air control valve (IAC).

I decided to tap on the IAC with butt-end of a screwdriver. The idea being that the valve may be caked with gunk, preventing it from opening. Tapping it could dislodge the gunk and allow a start.

And it worked! Car started right up and idled.

Pulled IAC valve and throttle body. There are two styles, mine is #2:



The IAC inlet and outlet tubes were completely caked in black solid deposit. Used throttle body cleaner, shop towels, and a toothbrush to clean it out. Did not disassemble to to lack of a good procedure - also noticed that a salvage unit appeared to be glued and was concerned about re-sealing. Also thoroughly cleaned the throttle body. Re-assembled with a new IAC gasket (#9) and a new throttle body gasket. #9 gasket is critical because coolant flows very close to air intake and the gasket is the only thing preventing coolant from entering intake.

Started up without problem. Burped coolant system (IAC is jacketed with coolant - probably to keep it from freezing shut in code temp - and some coolant leaked out). When warm, re-adjusted idle (it was too low) using FSM manual procedure.

It's been 3 weeks and dozens of hot starts. So far only two cases of sputtering start immediately following procedure. But nothing after that. Oddly enough, the procedure also seems to stopped most of the pinging that could previously be heard under load.

tl;dr: I was able to diagnose a dirty IAC valve by tapping on it with a screwdriver. Cleaning IAC and throttle body solved an intermittent hot hard start problem.

Jimi Hondrix

Thanks for posting the resolution.

As a sidenote: The IACV gasket can become flattened and rigid over time and develope a small air leak. I've added a small amount of Hondabond to an old gasket just to ensure no leaking..no issues.