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In short my engine jumps around between 400-900rpm. It also has trouble keeping itself on when cold starting and tapping the throttle will occasionally result in bogging or misfiring (only when cold). After a few good revs it'll settle down a bit and stay running on its own but will still be jumping around.
My current guess is something with the fuel system. I'm currently waiting to get a new fuel sending unit since the one I have now is bad (fuel gauge always reads full) but even with that I don't think that's the cause of the problem. I've gone through the trouble of replacing almost every sensor associated with the idle (tps/ecm/iac) but none of those made a difference. It doesn't seem to need a tuneup since all the plugs and wires are still fairly new. I've also gotten it tested for vacuum leaks but no luck there. I don't get a check engine light for this but when driving in a more spirited manner (the way you're supposed to) it will occasionally go on then off again randomly. When the check engine light does go on there isn't any noticeable difference in the way it runs and it's not low fuel since it happens on a freshly filled tank as well.
Any advice is greatly appreciated (unless you tell me to sell my car I hear that enough from my family).. THANKS!
First things first, have you checked the timing on the car, from experience mine did the same thing exact mine ended up being the harmonic balancer had spun and timing was way off! I also had vacuum leak which exasperated the issue!
New fuel filter and tune up for sure plugs wires cap and rotor, get FSM if not got one and test components instead of just replacing much cheaper
Thanks for the input! Unfortunately my only tools are my hands, a rag, the ratchet for the targa top, and a random hodgepodge of screwdrivers strewn about the house. I've had a few people tell me to check the timing so when I get the chance I'll take a look at it. As of now I'm in a bit of a waiting period while my wallet fattens up a bit but hopefully I can get this all sorted by the time it starts to get warmer out.
I'll probably be back with updates and hopefully it'll help out any others having similar issues.
Okay so the two most common issues that can cause an idle surge is over advanced timing and lean air/fuel ratio. Since you say it bogs down, I would say something is causing it to run lean. The first thing in this situation that I would suspect is the Coolant Temperature Sensor. You will need to connect a scan tool and find out what temp the ECM sees.
Second thing we could be dealing with is faulty injectors. Injectors are a common issue with these cars and almost all of the L98s need to have the injectors replaced at some point. The most common failure for these injectors is the electrical aspect. Use an ohmmeter on each injector and find out which ones read low. I believe if they read anything less than 12.5 ohms, they are no good. Typically you should expect to see 14-16 ohms for a normal injector. However, from my experience I have usually seen bad injectors like this cause the engine to run rich. But there is no definitive evidence that I have come across that would ever lead me to believe that they could not cause the engine to run lean.
Next thing could be your standard misfire. A slight ignition misfire on these cars could cause the engine to surge, an example of a slight misfire would be a weak or broken wire insulation for the spark plug wire.
I would start looking in those areas before proceeding any further. You may also want to pull the spark plugs out and see if any of them are fouled, which would be indicative of a misfire or running too rich. I don't think you are running too rich in this case.
Well now that you mention coolant temperature sensors, it does seem like mine has seen better days.
This seems like an easy fix but at the same time I feel like I still need to inspect the other possible causes as well. I'll order a new sensor for now and see where that gets me.
That is not the CTS. That is the Temperature Sending Unit. The sensor in the picture sends data to the digital dash cluster, not to the ECM. You need to investigate the one for the ECM which is in the front of the intake manifold. You also need to confirm that you are getting an accurate reading with the ECM.
That is not the CTS. That is the Temperature Sending Unit. The sensor in the picture sends data to the digital dash cluster, not to the ECM. You need to investigate the one for the ECM which is in the front of the intake manifold. You also need to confirm that you are getting an accurate reading with the ECM.
My mistake! I knew there were 2 temp sensors but wasn't sure which one was which. When I get the chance I'll check the other one.
Quick question though. If that sensor was bad would it still give me temps in the gauge or is it a completely different thing to have the gauge tell you the temperature and for the sensor to actually be doing what its supposed to?
The temp sensor in the picture sends the temperature to the dash (nowhere else). The temp sensor on the front of the intake manifold is ONLY used by the ECM.
As it turns out I overlooked a pretty basic issue (dirty throttle body and intake). Now that its clean the car runs great. I also got a new thermostat and fiddled with the wiring for the coolant fans since they weren't turning on when they were supposed to. Thanks again for all the advice!
02-02-2016, 12:51 AM
1985 Corvette jumping idle
