High idle and occasional difficulty in starting

[br1anstorm]

I have a 1993 XJ with 4.0 H.O. engine (ie non-Renix). Owned since new, no engine modifications, has had new ignition coil but few other replacement engine-parts aside from routine filters etc. Mileage now 178,000, but it has not been heavily used off-road.

Until recently the engine has run reliably and flawlessly, starting first time every time (it has a good Optima battery).

Recently two slight problems have appeared. I don't know if they are related. Only sometimes, the engine is reluctant to start... churns a few times and then usually fires if the throttle pedal is pushed to the floor. Feels slightly as if it's either not getting enough fuel, or not enough spark.

Also, once running the idle speed seems to be higher than normal - at least until the engine is warm. As I recall it has always idled solidly and reliably at about 700 rpm ever since it was new. Now, sometimes but not always, it is 1000-1100 rpm. Blipping the throttle pedal doesn't necessarily bring it down.

I don't have OBD scanners or similar diagnostic tools. So far, I have only done some simple basics:

• put in some clean quality fuel, in case I had a bad tankful;
• cleaned the inside of the distributor cap (whose contacts were a bit dirty/corroded);
• checked and cleaned all the spark plug leads;
• checked that the throttle pedal or cable isn't catching or sticking in the footwell carpet.

The symptoms remain - but are intermittent. I have browsed the forum and there are hundreds of 'high idle' posts and hundreds of possible explanations! This post from Yellaheep at https://www.naxja.org/forum/showthread.php?t=965655 seems to be close to my situation, but I'm still not clear about how to troubleshoot. I can't throw replacement parts in randomly (I'm in UK, no junkyard options and spares are like gold dust). What other checks can I do and - importantly - in what order, in order to narrow down the cause(s) of the problem(s)?

 

[RCP Phx]

I would suspect a bad IAC or a vacuum leak but you should never have to touch the throttle to start it.

 

[8Mud]

You can try what I do.... Spray a few squirts of petroleum based brake cleaner into the IAC intake with the motor running, blip the throttle to avoid a stall and flush the sludge out. All brake cleaners aren't made the same, you need one that dissolves sludge well. I just bought a case of Presto brake cleaner, Amazon special, stuff is useless.
Quick start (ether) dissolves sludge well, but it is unlikely you should use it with the motor running.

 

[br1anstorm]

Thanks for those helpful replies.

Looks as if the first, or next, things to do are (1) to check for vacuum leaks - I hope I can identify all the possible locations and hoses/pipes/connections!

And then (2) to check and probably clean out the throttle body and the IAC - on which there are several good YouTube videos if I need to seek guidance. Well, that's my weekend task.....

 

[Unclewolverine]

#1 diagnostic check when you have a throttle control issue should be the tps with a analog volt meter. Those damn things are so flakey and can cause so many different and weird problems. Yours could be going bad at the very beginning of the sweep.

 

[cruiser54]

Be sure to verify the intake manifold bolts haven't loosened up. I hope you're aware that plugs, wires, cap and rotor are due to be replaced every 30,000 miles.

a thorough throttle body cleaning is always a good idea.

 

[br1anstorm]

Thanks for the further comments. I've looked at the throttle body and IAC and plan to clean them anyway...

But I'm finding it difficult - as a non-expert - to figure out exactly where all the vacuum lines are before I start trying to check them. The diagrams and images I have found on line seem to suggest that the vacuum "network" of hoses varies remarkably not just between Renix engines and later ones, but differs from one model-year to the next.

Mine's a 1993, 4.0 litre fuel injection High Output engine. The MAP sensor is not on the throttle body, it's fixed to the firewall. Can anybody point me to either a clear parts/workshop diagram which shows all the vacuum lines and connections, or a photo which does so? I need to know exactly where to check before I start messing with flammable gases or liquids!