86 C4 starts, runs, rough idles and sometimes stalls

M

monsolo2

Member
Grumpy, I just bought a nearly flawless (interior & exterior) 1986 C4 coupe. When I drove it home from the dealership, I noticed a slightly rough idle but still good performance out of the engine. First thing my son and I did was replace the plugs, wires, distributor, and rotor... started her up and she purred like a kitten until it kicked down into low idle ~600rpm. The engine then lost rpm until it finally died.

I'm thinking the problem might be in the throttle body somewhere... I'm also going to check the full pressure as well in order to rule out a fuel problem. I'm just trying to zero in on a possible culprit as quickly as possible since I really want to drive it this weekend. Also, the "Service Engine Soon" light has come on a couple of times while driving today BUT I got no codes out of the computer when I jumpered the terminals this evening! It just keeps blinking "12" continuously...

Car only has 66k miles on it, by the way.

Thanks!

Derek
 
Possible faulty ignition module.
Possible faulty HEI Distributor pickup coil.
Possibly faulty HEI ignition coil.
Possible Power source problem to Ignition and fuel injection.
Possible plugged fuel filter.
Possible faulty fuel pump.
Need to test all. Verfiy problem .
 
then clean your IAC and adjust your TPS, clean throttle body and check for loos vacuum hoses and other vacuum leaks

http://garage.grumpysperformance.com/index.php?threads/adjusting-your-tps-and-iac.168/

it sure sounds like the throttle body or IAC is dirty and needs to be cleaned, and TPS sensor set,I'd also suggest replacing the fuel filter as they can cause issues when partly clogged, the links below should prove useful.
verify your fuel pressure is steady at about 38-40 psi and the fuel rail pressure stays that high for at least 5-7 minutes after the engines shut off.
this helped verified the fuel pump and regulator function and helps eliminate potential injector related issues.
as always you verify compression on all 8 cylinders ,verify the ignition,advance curve and spark,verify fuel pressure,stays consistently at near 38-40psi and that fuel delivery if maintained.
pull trouble codes and verify the fuel rail pressure and look for defective sensors, set the timing,get a set of NOID TEST LIGHTS, and verify the injector pulse at each location, and then youll have a realistic base to trouble shoot from
IF YOUR SIMPLY TRYING TO GET A STOCK TPI CORVETTE ENGINE BACK IN SERVICE AS CHEAPLY AS YOU CAN?
the heads and block surfaces must be very carefully examined for damage or warping issues and if found those issues must be corrected, before any new head gaskets installed, over time steam can and will cut grooves in even cast iron blocks and rather easily in softer aluminum. no head gasket will seal a badly machined or warped head or block
btw if youve managed to blow a head gasket on a 1986-91 TPI corvette with aluminum heads
the heads and block surfaces must be very carefully examined for damage or warping issues and if found those issues must be corrected, before any new head gaskets installed, over time steam can and will cut grooves in even cast iron blocks and rather easily in softer aluminum.
keep in mind coolant must have the anti-freeze ratio set at 50%water 50% antifreeze and be replaced at least every 4 years MAX to retain its anti corrosive additives
fel-hs7733pt9_xl.jpg

fel-hs7733pt9_xl.jpg

the stock 1986-91 tpi head gasket FELPRO HS7733pt9

buy a CHEVY SHOP MANUAL FOR YOUR YEAR CORVETTE!
you always need a base line to start from, on a TPI corvette.
a logical step by step approach and keeping accurate notes helps.
youll NEED a multi meter, a shop manual and a timing light and fuel pressure gauge at a minimum,
set and verify your ignition timing, pull trouble codes,set your tps and iac,, then check for vacuum leaks on the lines and intake,then get out your multi meter and verify all the sensors, chances are good its a defective heat sensor or defective o2 sensor, a logical step by step approach will lead you to the problem, youll be amazed at what youll learn reading links. use of a shop manual and multi meter can be very helpful
1996_Corvette.jpg

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http://www.harborfreight.com/5-in-1-dig ... 98674.html
put a fuel pressure gauge on the fuel rail on the shrader valve,

2713s.jpg

a vacuum and fuel pressure gauge is useful
tpishrader.jpg

fuelcirc1.png

L98/ TPI Engine Start Sequence
When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure. The reading will go to 38-40 pounds nominal once the engine is running.

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender. It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. (In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":

When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.

Summary:

If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.
youll find a hour or so reading thru the links and sub links, on this site in the threads, will provide a great wealth of related info and incite into related factors, or the function or testing of sensors, that you may not currently be thinking about, or things that you might not think that are related to your issue that PROBABLY ARE


reading the links and sub links will prove rather useful, now and even more so later
 
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Guys, thanks for the info! It's just good to have feedback simply becasue it gives me encouragement to jump in and problem solve. I'll check the fuel pressure first and then work my way to the throttle body and clean it and then check the TPS and other things.
 
let us know what you find and if possible post a few clear pictures of the process, and car.
youll find a hour or so reading thru the links and sub links will provide a great wealth of related info and incite into related factors, or the function or testing of sensors, that you may not currently be thinking about, or things that you might not think that are related to your issue that PROBABLY ARE
without testing you simply guessing
never guess, deal in verified FACTS!
guessing is a waste of time
think logically, isolate and test
yes IM aware some guys would rather dig out their eye with a red hot fork than to read links, but if you take the time to actually research the problem and isolate and test you,ll soon find the answers
well the first thing you do is pull trouble codes
and it would help if you purchased a shop manual.
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logically youll want to verify the fuel pressure is consistent and theres spark at the spark plugs, obviously when its starts and runs you have both fuel pressure and ignition spark, and when it won,t start you have lost one of the two.

start by pulling trouble codes, and get a fuel pressure gauge and a multi meter, if you own a vette you NEED that and a timing light and a SHOP MANUAL
Id suspect a defective sensor, and the oil pressure sensor near the base of the distributor is suspect, as is the ignition and all the electrical ignition connections and components, and perhaps the vats system.

use of a shop manual and multi meter can be very helpful
READ THE LINKS AND SUB LINKS
in the long term its going to provide you with a wealth of info youll need badly

http://garage.grumpysperformance.com/index.php?threads/c4-c5-corvette-trouble-codes.2697/

http://garage.grumpysperformance.co...start-right-back-up-and-run.10739/#post-46893

http://garage.grumpysperformance.com/index.php?threads/tpi-fuel-pressure-issue.10385/#post-42943

calculate fuel injector size

https://www.corvettemods.com/C4-C5-Corvette-1985-2004-Fuel-Injectors---Set-of-8---96lbhr---1000cc_p_17958.html?gclid=EAIaIQobChMIq8HDsqO69gIVoQaKAx1uHwGmEAEYASABEgKWQPD_BwE be sure you calculate the correct size injector and be sure you order the correct size injectors you need be sure to...
garage.grumpysperformance.com

http://garage.grumpysperformance.co...ouble-shooting-rebuilding-hei-ignitions.2798/

http://garage.grumpysperformance.co...years-now-i-want-to-sell-it.13790/#post-70731

http://garage.grumpysperformance.co...ard-starting-tpi-crossfire-or-lt1-vette.1401/

http://garage.grumpysperformance.co...lay-switch-locations-and-info.728/#post-54562

vette won't fire up after sitting

1989 sitting for 15 years - Crank but no start 1989 C4 ... 142K miles. Sat indoors for 15 years. Cranks but will not start. Drained old gas, replaced fuel pump, sending unit, fuel filter. Fuel pressure is a solid 45psi with the pump is running, drops to 40psi when pump is not running. There...
garage.grumpysperformance.com

http://garage.grumpysperformance.com/index.php?threads/adjusting-your-tps-and-iac.168/

http://garage.grumpysperformance.co...-system-trouble-shooting-flow-chart-info.596/

http://garage.grumpysperformance.co...-system-vats-making-you-crazy.520/#post-45062

http://garage.grumpysperformance.co...s-cause-a-bad-idle-in-drive.14203/#post-72114

http://garage.grumpysperformance.co...questions-can-be-found-here.12892/#post-67808
 
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Will do! Since you've last posted, I've been doing a LOT of reading, here and on various other sites. I'm convinced it's the IAC so I'm changing it out today... chances are, it's also 28-years old. I'm also going to change out the coil as well... :geek:

When I'm done, the fuel pressure will get checked. I'll let you know how all of this goes... :mrgreen:
 
Ok... after MUCH struggle and cursing of GM engineers, I managed to get the IAC replaced and set according to procedure. The biggest question out of all of this? Where can I find replacement plastic connectors??? The connectors on this car are so old that they are extremely brittle and I broke two while just trying to lift up the retainer to pull them off! In fact, one of the connectors that broke is the MAF sensor connector... I think that's why the ECM is now throwing a code 34 and 42! The other connector is the IAC...

Argggggg! :x
 
the search feature, is always an option here, on this site, but to save time , look at the sub links in these threads, to find sources for replacement electrical connectors, and NAPA can frequently ORDER replacement connectors for repairs at about 3- 5 times the cost youll find them at else ware, but at times getting the part the next day beats waiting a week so the price may be justified. if its a connector thats likely to break frequently buy extras, and have them handy
jktucker92 said:
There are books written out there that can give you the advantages / disadvantages of each system, but I'll try to keep it brief. In order to run as efficient as possible, you need to mix fuel and air at a specific mass ratio. 14.7:1 is the Stoichiometric ratio that is ideal, but richer mixtures can provide more power. With a MAF sensor, you measure the mass of the air flowing in the intake, which makes the calculation of how much fuel simple and accurate. The problem is the MAF sensors are more expensive than a simple pressure and speed sensor, especially early on. Also, the MAF sensors can be restrictive when you want to increase performance, so they are often removed in high performance applications. A speed density system uses the speed of the engine, manifold pressure, and temperature to calculate the mass of the air flowing into the intake and into the cylinder. This is pretty accurate, but not as accurate as a MAF sensor. As a result, the engines generally are tuned to run a little richer than the MAF systems to avoid damaging the engine by running too lean all the time.
Whether an engine is batch or sequential injected is a different, but related topic. In order to have sequential injection, the engine must be port injected, and the injector fires on each cylinder as each valve is opened. Batch systems can fire all the injectors on each revolution, or half one one revolution and the other half on the next. As it comes out of the injector, it's a fine mist, and the longer it is in the manifold, the more that mist becomes larger droplets, which burn less efficiently. Port injected systems are better than throttle body systems because their injectors are close to the ports and the fuel stays in the fine mist better.
The most efficient system is a MAF sequential injected system, which is why all new vehicles are MAF sequential injected systems.
Click to expand...

vette won't fire up after sitting

1989 sitting for 15 years - Crank but no start 1989 C4 ... 142K miles. Sat indoors for 15 years. Cranks but will not start. Drained old gas, replaced fuel pump, sending unit, fuel filter. Fuel pressure is a solid 45psi with the pump is running, drops to 40psi when pump is not running. There...
garage.grumpysperformance.com
 
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Thanks, Grumpy... I'm ordering new connectors today. I'm also going to reconnect the two connectors and figure out a way to keep them secured temporarily. I'm also going to remove the negative battery cable and clear the ECM and then drive her around and see how she performs.

More to come... :ugeek:
 
zip strips seem to get the job done for me when i need a fast fix while waiting for replacement parts
 
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